© 2016 emma l. mortensen all rights reserved

© 2016

Emma L. Mortensen

ALL RIGHTS RESERVED

INVESTIGATION OF ELECTRODEPOSITED CUPROUS OXIDE THIN FILMS

by

EMMA L. MORTENSEN

A dissertation submitted to the

Graduate School- New Brunswick

Rutgers, The State University of New Jersey

In partial fulfillment of the requirements

For the degree of

Doctor of Philosophy

Graduate Program in Materials Science and Engineering

Written under the direction of

Dr. Dunbar P. Birnie, III

and approved by:

______________________________

______________________________

_______________________________

_______________________________

New Brunswick, New Jersey

October, 2016

ii

ABSTRACT OF THE DISSERTATION

Investigation of Electrodeposited Cuprous Oxide Thin Films

By EMMA L. MORTENSEN

Dissertation Director:

Dr. Dunbar Birnie, III

This dissertation focuses on improvements to electrodeposited cuprous oxide as

a candidate for the absorber layer for a thin film solar cell that could be integrated into

a mechanical solar cell stack. Cuprous oxide (Cu2O) is an earth abundant material that

has a bandgap of ~2 eV with absorption coefficients around 102-106 cm-1. This bandgap

is not optimized for use as a single-junction solar cell, but could be ideal for use in a

tandem solar cell device. The theoretical efficiency of a material with a bandgap of 2.0

eV is 20%. The greatest actual efficiency that has been achieved for a Cu2O solar cell is

only 8.1%. For the present work the primary focus has been on improving the

microstructure of the absorber layer film. The Cu2O films were fabricated using

electrodeposition. A seeding layer was developed using gold (Au); which was

manipulated into nano-islands and used as the substrate for the Cu2O electrodeposition.

The films were characterized and compared to determine the growth mechanism of each

film using scanning electron microscopy (SEM). X-ray diffraction (XRD) was used to

establish and compare the chemical phases that were present in each of the films. The

iii

crystal structure of the Cu2O film grown on gold was explored using transmission

electron microscopy (TEM), and this helped confirm the effect that the gold had on the

growth of Cu2O. The Tauc method was then used to determine the bandgap of the films

of Cu2O grown on both substrates and this showed that the Au based Cu2O film was a

superior film. Electrical tests were also completed using a solar simulator and this

established that the film grown on gold exhibited photoconductivity that was not seen

on the film without gold. In addition, for this thesis, a method for depositing an n-type

Cu2O film, based on a Cu-metal solution-boiling process, was investigated. Three forms

of copper were tested: a sheet of copper, electrodeposited copper, and sputtered copper.

The chemical phases were observed using XRD, microstructure was examined using

SEM, and the electrical properties were tested using a hot probe test. The sputtered

copper turned out to be the most stable film so it was used as the n-type in a

homojunction solar cell with the p-type electrodeposited Cu2O. Recommendations for

future experimentation with Cu2O film development, to improve upon the films and our

understanding of the material are included.

iv

ACKNOWLEDGMENTS

I am incredibly grateful for the support that I received from my advisor Dr.

Dunbar Birnie. The trust that he put in me and the guidance he gave me throughout this

journey has given me the confidence to continue pursuing my goals and reach even

higher.

I would like to thank my committee members: Dr. Deidre O’Carroll, Dr. Lisa

Klein, Dr. Allen Bruce and Dr. Frederic Cosandey for their input, critique and evaluation

of my work.

Thank you to all of my department members that have been by my side through

classes and this research. I would specifically like to thank my fellow group members

past and present: Sean Langan, Ross Rucker, Josh Epstein, Vishnu Vijayakumar, and

Brian Viezbicke. I am very thankful for the undergraduate students that have put so

much of their time into this research, specifically: Jenny Coulter, Aishwarya Limaye,

Joseph Csakvary, Janki Patel and Tara Nietzold. I would also like to thank all the

undergraduate students that have passed through DELLC that I have had the pleasure of

mentoring, their excitement for their new journeys in engineering has inspired me more

than anything else.

Thank you to Rutgers Materials Science and Engineering Department for the

funding I received during my time here from the McClaren and the IGERT Fellowships.

Specifically, I would like to thank Dr. Lisa Klein for her assistance throughout my time

at Rutgers, and Nahed Assal, Claudia Kuchinow, and Sheela Sekhar for their help with

the administrative side of obtaining this degree.

v

I would like to send a very special thank you to my parents, sisters, grandparents

and all of my friends that have stood by my side throughout this journey. I would like

to thank them all for their support, patience and belief in me. Finally, I would like to

thank my husband Patrick for his unconditional love and support, especially through

these final steps, his unwavering confidence in me has made all the difference.

vi

TABLE OF CONTENTS

ABSTRACT OF THE DISSERTATION ........................................................... II

ACKNOWLEDGMENTS ................................................................................ IV

TABLE OF CONTENTS ................................................................................. VI

TABLE OF FIGURES ....................................................................................... X

LIST OF TABLES ........................................................................................ XIV

CHAPTER 1 ........................................................................................................1

1. Introduction ................................................................................................1

1.1 Global Energy Statistics .........................................................................1

1.2 Background of Photovoltaics ..................................................................4

Energy Loss ......................................................................................4

Thin Film Photovoltaics ...................................................................7

Multi-Junction Solar Devices ...........................................................8

1.3 Cuprous Oxide Solar Cells ...................................................................11

Heterojunction Cu2O Solar Cells ...................................................12

Homojunction Cu2O Solar Cells ....................................................13

1.4 Goal of Presented Work .......................................................................15

1.5 Breakdown of Thesis Work ..................................................................15

vii

CHAPTER 2 ......................................................................................................17

2. Electrodeposition of Cuprous Oxide ........................................................17

2.1 Background of Cuprous Oxide thin film fabrication ............................17

2.2 Experimental Reagents .........................................................................19

2.3 Deposition of Cuprous Oxide ...............................................................19

2.4 Characterization ....................................................................................20

Scanning Electron Microscopy ......................................................20

X-Ray Diffraction ..........................................................................21

2.5 Results and Discussion .........................................................................23

CHAPTER 3 ......................................................................................................25

3. Orientation Analysis of Cuprous Oxide Seeded with Gold Nano-

Islands 25

3.1 Overview ..............................................................................................25

Background of Gold and Cuprous Oxide .......................................26

3.2 Deposition of Gold Nano-Islands .........................................................29

3.3 Deposition of Cuprous Oxide on Gold .................................................30


Scanning Electron Microscopy ......................................................31

X-Ray Diffraction ..........................................................................34

viii

Transmission Electron Microscopy ................................................37

Solar Simulation .............................................................................42


CHAPTER 4 ......................................................................................................46

4. Optical Properties of Cuprous Oxide .......................................................46

4.1 Background of Bandgap Estimation .....................................................46

Cu2O Band Structure ......................................................................47

4.2 Tauc Method .........................................................................................49

4.3 Cu2O Data Collection ...........................................................................50


CHAPTER 5 ......................................................................................................56

5. n-Type Cu2O ............................................................................................56

5.1 Background of n-Type Cu2O ................................................................56

5.2 Experimental Reagents .........................................................................57

5.3 Substrate Preparation ............................................................................57

Copper Sheet ..................................................................................57

Electrodeposition of Copper...........................................................58

Sputter Coating of Copper .............................................................58

ix

5.4 Conversion from Cu to Cu2O ...............................................................59


Hot Probe Test ................................................................................59

Solar Simulation .............................................................................60

SEM ................................................................................................60

XRD ...............................................................................................63


CHAPTER 6 ......................................................................................................67

6. Conclusions ..............................................................................................67

CHAPTER 7 ......................................................................................................72

7. Future Work .............................................................................................72

7.1 p-Type Cu2O .........................................................................................72

7.2 n-Type Cu2O .........................................................................................74

7.3 Solar Cell Fabrication ...........................................................................74

BIBLIOGRAPHY .............................................................................................76

x

TABLE OF FIGURES

Figure 1.1 The global power capacity for photovoltaics. ................................................2

Figure 1.2 NREL research photovoltaic efficiencies records categorized by solar cell

type ..................................................................................................................................3

Figure 1.3 Intrinsic efficiency limit for single junction solar cell. ..................................5

Figure 1.4 Comparison of energy losses from narrow bandgap materials (left) and

wide bandgap materials (right) ........................................................................................6

Figure 1.5 Cadmium Telluride thin film photovoltaic architectural illustration. ............7

Figure 1.6 Basic multi-layer solar cell architecture ........................................................9

Figure 1.7 Intrinsic efficiency limit of 4-terminal mechanical stack tandem solar

cell. ................................................................................................................................10

Figure 1.8 Schematic of four-contact tandem solar device with a wide bandgap solar

cells on top and silicon solar cells on bottom ................................................................11

Figure 2.1 SEM images of Cu2O deposition using electrolyte 1 (top row) and

electrolyte 2 (bottom row) electrodeposited FTO coated glass for five minutes, and one

hour respectively. All images are at the same size scale. ..............................................21

Figure 2.2 XRD patterns for electrolyte 1 electrodeposited for 1 hour on FTO coated

glass ...............................................................................................................................22

Figure 2.3 XRD patterns for electrolyte 2 electrodeposited for 1 hour on FTO coated

glass ...............................................................................................................................23

Figure 3.1 Electrodeposited Cu2O deposited on FTO coated glass. .............................26

Figure 3.2 TEM characterization of a single Au/Cu2O core/shell face-raised cube. ....28

xi

Figure 3.3 FESEM image of gold nano-islands on FTO coated glass, with an average

size of 200nm ................................................................................................................29

Figure 3.4 SEM images of Cu2O electrodeposited, using electrolyte 1, onto different

substrates FTO (top), and gold nanoisland coated FTO (bottom). ................................32

Figure 3.5 SEM images of Cu2O electrodeposited, using electrolyte 2, onto different

substrates FTO (top), and Au nano-island coated FTO (bottom). .................................33

Figure 3.6 XRD pattern comparing electrodeposited Cu2O on an FTO coated glass

substrate (top) and on a gold coated FTO coated glass substrate (bottom). This sample

used electrolyte 1 ...........................................................................................................35


substrate (top) and on a gold coated FTO coated glass substrate (bottom). This sample

used electrolyte 2 ...........................................................................................................36

Figure 3.8 HR-TEM showing the cross section of the electrodeposited Cu2O on gold

nano-islands and FTO coated glass. The platinum is present from the process of

cutting the sample using FIB. ........................................................................................38

Figure 3.9 HR-TEM magnified image of a gold particle that has Cu2O electrodeposited

on top. The black box highlights the region where a fast Fourier transform was

taken. .............................................................................................................................39

Figure 3.10 Fast fourier transform of the area highlighted in figure 3.9. The arrows

highlight the spots that represent Cu2O and Au in the pattern. .....................................40

Figure 3.11 Inverse fast Fourier transform of the interface between a gold particle (left

side) and Cu2O film (right side) this is from the boxed area highlighted in figure 3.9 41

xii

Figure 3.12 Solar simulation data for electrodeposited Cu2O on an FTO substrate in

light. ...............................................................................................................................42

Figure 3.13 Solar simulation data for electrodeposited Cu2O on an FTO substrate in

dark. ...............................................................................................................................43

Figure 3.14 Solar simulation data for electrodeposited Cu2O on a Au/FTO substrate in

light. ...............................................................................................................................43

Figure 3.15 Solar simulation data for electrodeposited Cu2O on a Au/FTO substrate in

dark. ...............................................................................................................................44

Figure 4.1 The band structure of Cu2O and density of states from DFT

calculations(above) and the associated Brillouin zones (below) ..................................48

Figure 4.2 Energy-band diagram of Cu2O around the Γ-point. .....................................49

Figure 4.3 Tauc plot for the electrodeposited Cu2O on FTO coated glass, for

deposition times of 30 seconds, 1 minute, and 2 minutes. ............................................52

Figure 4.4 Tauc plot for the electrodeposited Cu2O on gold coated FTO coated glass,

for deposition times of 30 seconds, 1 minute, and 2 minutes. ......................................54

Figure 5.1 SEM image of a copper sheet ......................................................................61

Figure 5.2 SEM image of a copper sheet that has been boiled in copper sulfate to

convert it to Cu2O ..........................................................................................................61

Figure 5.3 SEM image of electrodeposited copper on FTO coated glass that has been

converted to Cu2O by boiling in copper sulfate. ...........................................................62

Figure 5.4 SEM image of sputtered copper on glass that has been converted to Cu2O

by boiling in copper sulfate. ..........................................................................................62

Figure 5.5 XRD data of Cu2O resulting from a copper sheet boiled in copper sulfate. 63

xiii

Figure 5.6 XRD data of Cu2O resulting from electrodeposited copper onto FTO coated

glass and then boiled in copper sulfate. .........................................................................64

Figure 5.7 XRD data of Cu2O resulting from a sputtered copper onto glass and then

boiled in copper sulfate. ................................................................................................64

Figure 7.1 Set up for photocurrent measurements. .......................................................73

xiv

LIST OF TABLES

Table I Literature review of Cu2O solar cells and their electrical

properties.........................14

Table II Summary of reported bandgaps of Cu2O and the authors that reported the

bandgaps. .......................................................................................................................46

Table III Possible transition types and the corresponding n values ..............................50

1

CHAPTER 1

1. Introduction

1.1 Global Energy Statistics

As of 2005 energy consumption for the world was at a steady 13 terawatts with

85% of that energy from fossil fuels. The use of fossil fuels has led to an increase of

carbon dioxide in our atmosphere, and we have reached the highest CO2 level in 125,000

years.[1] Without intervention these levels will continue to grow and become unstable

and unsustainable by 2050.[2] The U.S. Energy Information Administration has

predicted that the world’s energy consumption will increase by 56% from 2010 to 2040.

This growth is coming primarily from non-OECD countries.[3] Due to this projected

growth, and the global concern of climate change, there has been an increased push

toward renewable energy sources. As of 2012 21% of world energy generation came

from renewable sources such as solar, geothermal, and wind.[4] This use of renewable

energy is predicted to increase to 25% by 2040.[3]

2

Figure 1.1 The global power capacity for photovoltaics.[5]

Solar energy is on the rise. There is less backlash about the geopolitical,

environmental, concerns from the public for this energy source compared to other

renewable sources like nuclear or wind. As efficiencies rise and costs lower, solar is

becoming a more viable option. As of 2014 there was a capacity of 177 GW for

photovoltaics worldwide, this is almost 50 times more global power capacity compared

to that of the capacity in 2004 (figure 1.1).

Every hour 100,000 TW of solar power hits the earth’s surface; this is enough

energy to cover every person’s energy requirements for a year.[6] Low cost, and highly

efficient solar cells are required to use this sun light effectively. Due to price decreases

in panels, solar energy usage has increased; it now covers 1% of the global energy

3

demand.[7] Solar energy is still only a small part of the global energy sector, which

suggests there is room for growth, and that improvements still need to be made.

The National Renewable Energy Laboratory report includes efficiency records

that have been set in the different areas of solar cell research: multi-junction cells, single-

junction gallium arsenide cells, crystalline silicon cells, thin film cells, and emerging

PV. This data is depicted in figure 1.2

Figure 1.2 NREL research photovoltaic efficiencies records categorized by solar

cell type[8]

The NREL report includes data from concentrator (CPV) and non-CPV devices.

The CPVs use concentrating optics that allow for solar cells with reduced area.[9]

Concentrators are used when the film material is expensive and/or not earth abundant.

These cells have the highest reported efficiencies from NREL, but in addition to their

expense, these materials are also commonly toxic. The focus of this research is to

4

develop another option that uses non-CPVs with earth abundant materials, that is

competitive with cost and efficiency.

1.2 Background of Photovoltaics

Photovoltaic conversion requires a number of steps: first, a photon from the sun

is transferred to the photovoltaic device; then the photon is absorbed into the cell and

induces a transition across the bandgap of the semiconductor material that makes up the

solar cell; at this point two groups of charge carriers, electron-hole pairs, are created and

extracted to separate contacts where the electrical circuit is completed. The electron-

hole pair can also recombine and create a phonon that will result in heat. To create a

single junction solar cell four parts are required: a front contact, a p-type material, an n-

type material, and a back contact. A p-type material results from a deficiency of valence

electrons resulting in holes, and an n-type material results from extra electrons in the

conduction band. This can be an intrinsic characteristic of the material or a result of

doping. The p- and n-type material can be the same material with different dopant or

two different materials. When the p-type and n-type materials come in contact this is

called a p-n junction, at the interface a depletion region is formed where some of the

electrons from the n-type fill in some of the holes in the p-type and this creates an electric

field so current can flow.

Energy Loss

Unfortunately, due to the broadness of the solar spectrum there are intrinsic

energy losses in a solar cell that cannot be corrected. As a result of these losses, the

5

maximum efficiency that can be reached for a solar cell is around 35%, as illustrated in

figure 1.3. This maximum is known as the Schokely-Quiesser (SQ) limit and this exists

because of the trade off between current and voltage.

A material with a narrow bandgap will have energy loss resulting from relaxation

to band edge, where the photon energy exceeds the bandgap energy, so all the excess

photon energy is lost and converted to heat. This type of material has a larger current

and a lower voltage. A material with a wide bandgap has energy loss due to below

bandgap photons, where the photon energy is less than the bandgap energy and the

Figure 1.3 Intrinsic efficiency limit for single junction solar cell.[10]

photon is not absorbed. This type of material in a solar cell has a higher voltage and

lower current. These energy losses can be seen in figure 1.4. The balance of the voltage

and current occurs at the maximum point of the SQ limit at a bandgap of 1.1 eV, and

this is the bandgap for silicon. Silicon solar panels are the most common and

6

commercially available, this type of panel has a theoretical efficiency around that

maximum of 35%, however the actual efficiency of these panels has been reported at

around 25%, which is shown back in figure 1.2, represented by the solid blue squares.

Figure 1.4 Comparison of energy losses from narrow bandgap materials (left) and

wide bandgap materials (right)

The difference between the theoretical and experimental efficiencies is due to

losses other than the ones represented in figure 1.4, such as recombination.

Recombination commonly occurs at grain boundaries, which is a problem for

polycrystalline materials used in solar cells, particularly for thin film photovoltaics.[11]

The basis for the present research is to surpass the theoretical efficiency of a

single-junction solar cell by layering two solar cells and creating a tandem solar cell.

This would minimize the losses that are present in all solar cells because the layering

strategy will allow wider energy photons to be absorbed first and then the below

bandgap photons to be transmitted to the cell below thus limiting the loss from the

relaxtion to band edge. Toward this goal, this research has focused on developing the

7

top layer of this tandem solar cell, which is a thin film photovoltaic made from cuprous

oxide (Cu2O).

Thin Film Photovoltaics

Thin film solar cells have been investigated since the 1970s. The three most

common thin film photovoltaics are cadmium telluride (CdTe), copper indium gallium

selenide (CIGS), and amorphous silicon. Some benefits for this type of solar device are,

they can be made to be flexible, and they do not use as much material so they are cheaper

Figure 1.5 Cadmium Telluride thin film photovoltaic architectural illustration.[12]

to make. The maximum efficiency for a thin film solar cell as reported by NREL(2015),

and illustrated previously in figure 1.2, comes from a CIGS solar cell at 22.3%, made

by Solar Frontier.

8

The basic structure of a thin film solar cell consists of the four major parts

described earlier: back contact, p-type layer, n-type layer, and a top contact. An example

of a thin film solar cell is shown using an illustration of CdTe solar cell in figure 1.5. In

this image, the bottom is the back contact, this is traditionally a reflective metal film or

paste. This layer is used as a conduction pathway to complete the voltaic circuit. The

metal chosen as the back contact requires a work function that is compatible with the

semiconductor on top of it allowing for better flow of electrons. The next layer is the p-

type absorber layer, followed by the n-type window layer, these two layers form the p-

n junction. The p-n junction of a thin film solar cell is only a few microns thick or less.

In the case of the CdTe cell in figure 1.5 the n-type material is CdS and the p-type

material is CdTe. On top of the n-type layer is a transparent conducting oxide (TCO)

layer that is commonly a tin oxide doped with either fluorine or indium, this layer is the

top contact that completes the voltaic circuit. There is traditionally glass incasing this

solar cell.

Multi-Junction Solar Devices

A multi-junction solar device is created by stacking or growing two or more p-n

junctions on top of one another. In the configuration shown in figure 1.6, the bandgap

of the semiconductor junctions decreases from the top of the multi-junction device to

the back side. This configuration allows for the least amount of energy loss because the

larger energy photons are absorbed in the top cell, while any below bandgap photons

are transmitted and absorbed in the solar cell with the smaller bandgap. This layering of

9

different bandgaps results in a better use of the solar spectrum compared to a single

junction solar cell.[13]

Figure 1.6 Basic multi-layer solar cell architecture.[14]

There are two ways to create a multi-junction solar cell, the first is a monolithic

approach where a complete solar cell is made and then the next cell is grown epitaxially

on top of the first. With this type of stack there is a current matching constraint. The

second type of device is a series of mechanical stacked solar cells. In this case the solar

cells are made individually and then adhered together mechanically, there is no current

matching constraint in this setup. The most common multi-junction solar cell is made

of gallium arsenide (GaAs) in one or all of the layers. GaAs is expensive because it is

not an abundant material and it is also toxic, for this reason it is commonly used with a

solar concentrator so that less material is required.

10

Figure 1.7 Intrinsic efficiency limit of 4-terminal mechanical stack tandem solar

cell.[10]

For this research the focus is on a four-terminal mechanical tandem solar cell.

Cu2O is a wide bandgap material, which means it is not optimized for a single junction

device based on the SQ limit. However, when paired with silicon in a stacked-tandem

architecture, the wider bandgap Cu2O could be ideal. Figure 1.7 shows a simple

derivation of the expected total efficiency using a coupled SQ analysis of this stack, with

the maximum efficiency greater than 45% for this stacked design. The architecture

would require the top cell to have a bandgap around 2 eV with a bottom cell at 1.1 eV.

This stacked theoretical efficiency is based on achieving the individual theoretical

efficiencies for each of the two cells. The actual efficiency would be smaller, but the

possibility is there that the SQ limit for a single junction silicon solar cell would be

11

surpassed with the addition of the wide bandgap solar cell on top, and the price would

be reasonable due to the low cost of materials and processing. A single-junction silicon

Figure 1.8 Schematic of four-contact tandem solar device with a wide bandgap

solar cells on top and silicon solar cells on bottom

solar cell is completed by sealing the panel with a top glass window. To add the top

layer, the solar cell would be deposited onto the top glass and then this thin film solar

panel would be adhered onto the silicon panel. A schematic of this four-terminal

mechanical tandem solar cell is illustrated in figure 1.8.

1.3 Cuprous Oxide Solar Cells

The thin film photovoltaic device that is the focus of this research uses cuprous

oxide (Cu2O). It is believed that the first solar cell to be created was a Cu2O solar cell

made in 1839 by Edmund Becquerel. For this device he used two metal plates

submerged in a liquid to produce a continuous current when under sunlight.[15] The

conversion efficiency for this archaic solar cell was less than 1%. Cuprous oxide (Cu2O)

has been researched as a viable option for solar power converter due to its high

12

theoretical efficiency and low cost.[16-20] This theoretical efficiency has been determined

using the SQ analysis summarized graphically in figure 1.1 (page 4). Cu2O with its

reported bandgap of around 2 eV, would have a theoretical efficiency of about 20%,[19,21-

23] yet the experimental efficiencies for Cu2O solar cells to date have only achieved an

efficiency as high as 8.1%,[24] and before that the efficiency had peaked at 6%.[25] There

are two types of solar cells to consider, the first is a homojunction solar cell and the

second is a heterojunction solar cell. There have been many studies on both types of

solar cell for Cu2O. Some of the techniques for each kind of solar cell are described

below, and a summary of a variety of the Cu2O solar cells that have been reported on,

and their electrical properties, are found in table I.

Heterojunction Cu2O Solar Cells

Schottky barrier solar cells have been studied extensively with regard to

Cu/Cu2O and are the earliest example of the heterojunction solar cells using Cu2O.

These solar cells are traditionally produced thermally, or in solution, Iwanowski and

Trovich developed a technique that uses with hydrogen bombardment to reduce the top

layer of a Cu2O film to Cu.[26] This solar cell works when there is a metal in contact with

a semiconductor, at the interface of these two materials there is a highly resistant barrier

layer.[16]

In the studies of heterojunction Cu2O the most common n-type material is ZnO

or aluminum doped ZnO (AZO). ZnO is a good choice for the n-type material for this

cell because it has a wide bang gap of about 3.3-3.4.[27-30] The layers of this solar cell

are commonly electrodeposited one at a time on tin oxide (a TCO) coated glass.[27]

13

Oxidized copper sheets are also used to produce the Cu2O films, and the remaining

copper is used as the back contact, the n-type ZnO is then deposited using magnetron

sputtering.[31,32] Magnetron sputtering has also been used for the Cu2O layer along with

sputtering of ZnO to form the p-n junction.[33-35] Diindium trisulfide (In2S3), tin oxide

(SnO2), cupric oxide (CuO), and cadmium oxide (CdO) have also been reported to be

used as n-type materials with Cu2O.[36-39] The Cu2O solar cell with the highest efficiency

of 8.1% was reported by Minami, et al, and is a heterojunction solar cell with Zn1-xGex-

O n-type.[24]

Homojunction Cu2O Solar Cells

Homojunction Cu2O solar cells are less advanced than the heterojunction solar

cells, and this is due to less understanding and development of n-type Cu2O. It is

believed that the most promising path for increasing the efficiency of Cu2O solar cells

is with a homojunction solar cell.[17] There is some debate on the topic of intrinsic n-

type Cu2O,[40] however there are reports that n-type Cu2O has been created without

dopants.[18,19,41-44] There is also work showing the use of dopants such as fluorine,

chlorine, and bromine.[45,46] The efficiencies that have been reported for homojunction

Cu2O solar cells has not reached 1%.

14

14

Table I Literature review of Cu2O solar cells and their electrical properties. [23,24,27,31,32,37,39,43,44,47-53]

Author Front

Contact P-type (preparation) N-type

Back

Contact

PCE

(%)

Voc

(V)

Jsc

(mAcm-2) FF

Papadimitriou Al Cu2O (oxidized Cu) CdO Cu 0.4 2

Georgieva ITO Cu2O (electrodeposition) ITO 0.34 0.245

Katayama SnO2 Cu2O (electrodeposition) ZnO C paste 0.117 0.19 2.08 0.295

Minami (2004) AZO Cu2O (oxidized Cu) AZO Cu 1.21 0.41 7.13 0.42

Han ITO Cu2O (electrodeposition) Cu2O Cr/Au <0.1

McShane ITO Cu2O (electrodeposition) Cu2O Au/Pd 0.29 0.423 2.5 0.27

Minami (2011) AZO Cu2O (oxidized Cu) ZnO Cu 3.83 0.69 0.55

Hussain ITO Cu2O (electrodeposition) TiO2 In 0.15 0.35

Minami (2013) AZO Cu2O (oxidized Cu) Ga2O3 Au 5.38 0.8 9.99 0.67

Kim Cu2O (magnetron sputtering) Si 1.98 0.36 15.2

Jayakrishnan Ag Cu2O (electrodeposition) In2S3 0.377 0.118 0.33

Minami (2014) AZO Cu2O Ga2O3 Au 5 0.8 11.2 0.6

Hsu ITO Cu2O (electrodeposition) Cu2O (pH=4.9) Al 0.42 0.42 2.68 0.38

Tsai FTO/Pt Cu2O (chemical bath deposition) I electrolyte Cu 0.72 0.64 3.52 0.32

Elfadill ITO Cu2O (electrodeposition) ZnO Si/Pt 1.445 0.39 9.07 0.41

Minami (2016) MgF2/

AZO Cu2O (Na doped) (oxidized Cu) Zn0.38Ge0.62O Au 8.1 1.25 11.50 0.7

15

1.4 Goal of Presented Work

The main goal for this research is to investigate Cu2O as a possible wide bandgap

absorber layer for a mechanical tandem device. The focus was on producing and

characterizing a cuprous oxide thin film that can be used as one layer in a tandem solar

cell device. The thin film was deposited using electrodeposition, and the deposition

technique was modified to produce films that consist of columnar shaped grains. The

samples produced using this kind of deposition were characterized to describe their

microstructure, composition, optical and electrical properties. Exploration of n-type

cuprous oxide was also conducted. The thin film deposition was a two-step process, the

first step was a deposition of copper followed by a conversion process from copper to

cuprous oxide. This film was then characterized for information about the crystal

structure, composition and electrical properties. Solar cells were tested using the p-type

Cu2O on FTO, p-type on n-type, and n-type on p-type layers.

1.5 Breakdown of Thesis Work

Chapter 2 focuses on Cu2O deposition, and characterization of the resulting

films. Two electrolytes were deposited using electrodeposition at different times to

understand the deposition process and the grain growth from each solution. These films

were then characterized to determine the chemistry and crystal structure.

Chapter 3 discusses the process of microstructural manipulation using gold

nano-islands and the analysis of the orientations of the films with the influence of a gold

seed layer. These films were analyzed using a transmission electron microscopy (TEM)

16

scanning electron microscopy (SEM), and x-ray diffraction (XRD). The resulting data

and the analysis will be discussed.

Chapter 4 explores the optical properties of the cuprous oxide films specifically

focusing on the Tauc analysis to determine the bandgap of the material. Different

thicknesses were analyzed to determine if the thickness of the film had an effect on the

bandgap as well as the influence of the gold nano-islands on the bandgap.

Chapter 5 looks into the experimental procedures of depositing cuprous oxide in

an attempt to produce an n-type Cu2O layer. It also delves into the characterization of

these films to determine the growth process, chemical makeup and the electrical

properties of the films that were converted from copper to cuprous oxide.

Chapter 6 provides the conclusions that were drawn from this work.

Chapter 7 discusses the areas where future work might continue based on the

work presented in this thesis.

17

CHAPTER 2

2. Electrodeposition of Cuprous Oxide

2.1 Background of Cuprous Oxide thin film fabrication

As discussed in section 1.2.3, cuprous oxide (Cu2O) has been researched as a

viable option as a solar power converter since the invention of the solar cell, due to its

high theoretical efficiency and low cost.[16-20] Experimental efficiencies for Cu2O solar

cells to date have only achieved an efficiency as high as 8.1%.[24] Therefore there is a

lot of work that can be done to maximize the experimental efficiency to get closer to the

theoretical efficiency mark.

The original method for obtaining a film of Cu2O was thermal oxidation of

copper in air temperatures ranging from 1050oC[16,54] to 500oC.[47] Using this method

resulted in films of Cu2O as well as CuO depending on the annealing temperature; the

higher temperature was said to produce pure Cu2O on copper while using a lower

temperature between 200oC-970oC resulted in a top layer of the CuO film.[54,55] The top

layer of CuO had to then be ground off to leave a film of Cu2O on copper.[47] A similar

method from Rosenstock, et al described using a silver layer on top of copper to assist

in the diffusion of oxygen at a temperature of 500oC to obtain a layers Cu/Cu2O/Ag.

This resulted in an extremely thin film of Cu2O.[56]

Another technique commonly used to deposit a film of Cu2O is a sol-gel method.

In a paper from Ray, et al the deposition is a sol-gel dip technique where the substrate

18

is dipped into a methanolic solution of copper chloride (CuCl2).[57] Other uses of the sol-

gel method rely on using a spin coater to distribute the solution on a substrate.[58-60]

The most promising method for deposition of Cu2O is electrodeposition because

it can be controlled easily, and can be done cheaply. Electrodeposition can be

accomplished in a one or two electrode system, where the third electrode is a reference

electrode. The elements required for electrodeposition are a working electrode, which is

a conduction substrate, a counter electrode, which can be a piece of metal or carbon, a

voltage source and an electrolyte solution.[19,27,37,41,44,48,61-68] The first step of this

deposition process is the reduction of Cu2+ ions to Cu+ ions, which is shown in equation

1. The second step is the precipitation of these Cu+ ions to form Cu2O, and this occurs

because the ion is not stable. This step is shown in equation 2 and the combination of

equations 1 and 2 is shown in equation 3.[18]

Cu2+ + e- Cu+ (1)

2Cu+ + H2O Cu2O + 2H+ (2)

2Cu2+ + H2O +2e- Cu2O + 2H+ (3)

Dendritic branching from Cu2O crystals is common in electrodeposited samples.

The dendritic growth occurs when there is a high deposition rate that results in a region

of Cu2+ around a growing crystal of Cu2O.[69,70] Once this region is formed, the dendrites

will grow faster than the central grain.

The goal of this research is to optimize the growth of Cu2O as the p-type absorber

layer of a solar cell to be used as the top cell of a stacked tandem solar structure with

19

silicon as the bottom cell. Experimentation for this thesis has focused on improving the

microstructure of the Cu2O layer to improve the efficiency of a Cu2O solar cell.

2.2 Experimental Reagents

The chemicals that were used for this work are as follows: (i) copper (II) acetate

monohydrate, 98.0+% purity, Alfa Aesar purchased from Fisher Scientific; (ii) copper

(II) sulfate pentahydrate, greater than 98.0% pure, purchased from Sigma Aldrich; (iii)

sodium acetate trihydrate, 99% pure, SigmaUltra purchased from Sigma Aldrich; (iv)

sodium hydroxide, 2.5N solution (10%w/v), purchased from Fisher Scientific; (v)

sodium DL lactate solution syrup 60% (w/w), synthetic, Sigma Life Science purchased

from Sigma Aldrich; (vi) acetone ACS grade, purchased from Fisher Scientific.

2.3 Deposition of Cuprous Oxide

Electrodeposition was used to produce the Cu2O films. Before the deposition of

Cu2O the substrates were cleaned three times using an ultrasonicator with the substrate

in a solution of 2 vol % Micro-90 in DI water, DI water, and finally in acetone. Two

chemistries were investigated for the electrolyte solution. The deposition process

employed a two electrode, single compartment electrolytic cell that was connected to a

tunable voltage source. The working electrode was a piece of FTO coated glass, and the

counter electrode was a piece of platinum foil. The first electrolyte (E1) contained 0.02

M copper (II) sulfate pentahydrate, and 0.06 M sodium DL lactate, and six drops of

sodium hydroxide were added to bring the solution to a slightly acidic 6.31 pH.[71] The

second electrolyte (E2) was made up of 0.1 M sodium acetate trihydrate and 0.1 M

20

copper (II) acetate monohydrate.[72] The electrodeposition was performed at a potential

of 1.5 volts, for various times ranging from 30 seconds up to one hour.

2.4 Characterization

Scanning Electron Microscopy

To characterize these films the Zeiss Sigma Field Emissions scanning electron

microscope (SEM) was used to capture images of the surface of the films. Two

chemistries were compared after five minutes and after one hour of electrodeposition.

The comparisons can be seen in figure 2.1. E1 did not show the flowering microstructure

that was expected based on the results from Osherov et al.[71] The structure that resulted

from our experimentation was more of a cauliflower structure, however E2 did show the

more dendritic grains. We believe that the cauliflower structure was produced because

the voltage was higher than what is traditionally used causing overly rapid growth. Both

electrolytes produced even coatings.

21

Figure 2.1 SEM images of Cu2O deposition using electrolyte 1 (top row) and

electrolyte 2 (bottom row) electrodeposited FTO coated glass for five minutes, and

one hour respectively. All images are at the same size scale.

X-Ray Diffraction

X-ray diffraction (XRD) patterns were collected from coatings deposited from

E1and E2 on FTO coated glass for one hour of deposition time. The X’pert PANalytic

XRD was used for XRD data collection, all of the scans were performed from 20-45o2θ.

These patterns can be seen in figures 2.2 and 2.3. All patterns clearly showed the

formation of Cu2O. Trace amounts of copper (Cu) are also present in both XRD patterns.

In all cases the FTO substrate peaks are well represented (as noted on the graphs).

22

Figure 2.2 XRD patterns for electrolyte 1 electrodeposited for 1 hour on FTO

coated glass

For both patterns the most prevalent orientation for Cu2O are the (111) and (200)

directions. The most distinct difference between the patterns of E1 and E2 are the peak

heights. When you compare the highest FTO peaks there is a difference of intensity,

with E2 being greater by about 400, and for the highest Cu2O peaks there is a difference

of about 300 in intensity, again with E2 having the higher peak.

0

200

400

600

800

1000

1200

1400

20 25 30 35 40 45

Inte

nsi

ty

2θ

FTO

FTO

FTO

Cu2O

(111)

Cu2O

(200)Cu

Cu2O

23

2.5 Results and Discussion

Figure 2.3 XRD patterns for electrolyte 2 electrodeposited for 1 hour on FTO

coated glass

Cuprous oxide is a promising semiconductor material with likely value for future

tandem solar cells stacks. However, it has been held back in the past by low experimental

efficiency in spite of high theoretical expectations based on the bandgap.

Two different electrolytes were used and it was found that E2 was the more

desirable of the two, because the growth is more controlled. This electrolyte does grow

in a more dendritic/flower-like way and so further research focused on improving this

microstructure.

From the XRD patterns that were obtained from these samples it was established

that Cu2O was growing well on the FTO with orientation mainly in the (111) and (200)

0

200

400

600

800

1000

1200

1400

1600

1800

20 25 30 35 40 45

Inte

nsi

ty

2θ

FTO

FTO

FTOCu2O

(111)

Cu2O

(200)

Cu2O Cu

24

directions. Both electrolytes produced Cu, however, the ratio of Cu and Cu2O is smaller

in the E2 film.

25

CHAPTER 3

3. Orientation Analysis of Cuprous Oxide Seeded with Gold Nano-

Islands

3.1 Overview

In the previous chapter it was mentioned that the grains that grow when

electrodepositing Cu2O are primarily dendritic. This is not an ideal grain shape for an

absorber layer of a solar cell because it limits the through film pathways for electrons

and increases the number of grain boundaries. The density of the film is also a problem

with this grain shape; figure 3.1 shows a roughness between the petals of the grains and

this roughness is the layer of FTO that the film was deposited on. So the goal of this

work is to improve upon the microstructure of the films by focusing on the growth of

columnar shaped grains. To do that a technique was employed using a seed layer to

encourage growth across the film and in the vertical direction instead of the growth of

branches.

26

Figure 3.1 Electrodeposited Cu2O deposited on FTO coated glass.

Background of Gold and Cuprous Oxide

Gold and Cu2O are paired together frequently in research because of their similar

crystal structures. Cu2O is primitive cubic with a lattice parameter of 0.427nm, and gold

is face-centered cubic (fcc) with a lattice parameter of 0.4079nm. The lattice mismatch

between these two materials is about 4.7% when using equation 4.[73]

(afilm – asubstrate)/asubstrate (4)[73]

In other research gold has been used to seed the growth of one dimensional

nanostructures of materials like GaAs. To produce the nanowires they are grown from

the bottom up, a vapor method or solution based method is used to deposit the wire

27

material.[74] There are examples of a pairing Cu2O with gold in the literature in the form

of a gold substrate,[73,75] or as the core of a nanoparticle[76]. TEM analysis, from a paper

by Wang et al, on core/shell relationship between gold and Cu2O is shown below in

figure 3.2. In this image the (111), (110), and (100) are the orientations of the gold facets

and the zone axes that are showing are the [22̅2̅] and [22̅0]. In the images, a5 and b5

arrows point to the two diffraction points that represent the Cu2O crystal and the Au

crystal, they are adjacent to each other. This confirms that the structures and lattice

parameters are similar. Also in these images the gold particles are faceted and the Cu2O

matches those facets as the shell on top of the gold core. For this work gold has been

researched as a seed layer in the form of nano-islands that can be used in a similar way

to grow columnar grains.

28

Figure 3.2 TEM characterization of a single Au/Cu2O core/shell face-raised

cube.[76]

29

3.2 Deposition of Gold Nano-Islands

Gold nano-islands naturally form when a thin layer of gold is deposited on a

surface. Before the deposition of the gold, the glass was thoroughly cleaned as described

in Chapter 2 using an ultrasonicator three times with the substrate in a solution of 2

volume% Micro-90 in DI water, DI water, and finally in acetone.

Figure 3.3 FESEM image of gold nano-islands on FTO coated glass, with an

average size of 200nm

To obtain the gold nano-islands, gold was sputtered onto the FTO side of the

glass, using the EMS 150T ES. In order to expedite the formation of distinct nano-

islands the gold coated glass was baked. First the thickness of the gold was explored,

80nm, 50nm, 30nm, 15nm were tested and baked for six hours at 600oC. The 15nm gold

film resulted in the most uniform gold nano-islands. Then the bake time was tested, at

30

four hours, five hours and six hours. From this the six-hour bake was chosen because in

the earlier times the gold particles were more elongated, at six hours the particles were

the most consistent in size and shape. The resulting gold nanoparticles were an average

of 200nm and were evenly distributed on the glass. This can be seen in figure 3.3. The

FTO and gold nano-island coated glass was then used as the working electrode in the

electrodeposition.

3.3 Deposition of Cuprous Oxide on Gold

The electrodepositon set up for this experiment was the same as described in

Chapter 2, which consisted of a two electrode, single compartment electrolytic cell, but

in the case of the working electrode it was replaced with the gold coated FTO coated

glass. Again we looked at the two electrolytes E1 and E2. E1 contained 0.02 M copper

(II) sulfate, and 0.06 M sodium DL lactate, and 6 drops of sodium hydroxide were added

to bring the solution to a slightly acidic 6.31 pH.[71] E2 was made up of 0.1 M sodium

acetate and 0.1 M copper (II) acetate monohydrate.[72] The electrodeposition was again

performed at a potential of 1.5 volts for one hour.


The films were then analyzed and compared to determine the effects of the gold

nano-islands on the Cu2O films. The first characterization technique used was plan-view

SEM to understand the surface microstructure of the film. The next area that was

explored was the chemical phase identification using the XRD. The orientation

relationship between the Cu2O and the Au was also looked at using TEM. Finally the

31

electrical properties were compared between the film deposited on Au/FTO and the film

deposited on FTO

Scanning Electron Microscopy

The Zeiss Sigma Field Emission SEM was used to characterize these films.

Images were taken of the Cu2O samples for the two different electrolytes (E1 and E2)

that were deposited on the gold nano-island substrate and compared to those of the

samples of Cu2O deposited on the FTO substrate shown in Chapter 2. The comparisons

can be seen in figure 3.4 and figure 3.5. All images were taken at about 29KX

magnification.

When deposited on the gold, the Cu2O film from E2 no longer grew flowering

grains but faceted grains. E1 also showed faceted grains compared to the images taken

with the FTO substrate. For both electrolytes the grains grown on gold were significantly

smaller than those without, and the growth that did occur suggests a more controlled

growth when compared to the deposition on just FTO. The gold based growth also

encouraged a denser film growth because the rough area (known to be the FTO) that we

see in the film grown without the Au does not show in the SEM image of the film

deposited on Au.

32

Figure 3.4 SEM images of Cu2O electrodeposited, using electrolyte 1, onto

different substrates FTO (top), and gold nano-island coated FTO (bottom).

33

Figure 3.5 SEM images of Cu2O electrodeposited, using electrolyte 2, onto

different substrates FTO (top), and Au nano-island coated FTO (bottom).

34

X-Ray Diffraction

The X’pert PANalytic XRD was also used to compare the E1 and E2 films. As

mentioned in the previous chapter traces of copper were found in the films of both

electrolytes using the FTO substrate. When E2 was deposited on Au it was found that

the traces of copper had almost entirely disappear. However, the samples using E1

examined with XRD, show the copper traces were still present even when gold was

added to the substrate.

This comparison can be seen in figure 3.6 and 3.7. The top graph in both figures

is Cu2O deposited on FTO and the bottom graph is for Cu2O deposited on gold and FTO.

Also in both figures there is a decrease in the peaks of the Cu2O with the addition of

gold particularly in the peak representing the (111) orientation. This decrease in Cu2O

is due to a change in thickness when Au is added to the substrate, the film without Au

is not as dense and therefore the growth occurs in the vertical direction whereas the

growth with Au occurs from the substrate so it is not as thick and the resulting signal for

the XRD is not as strong.

35


substrate (top) and on a gold coated FTO coated glass substrate (bottom). This

sample used electrolyte 1

0

200

400

600

800

1000

1200

1400

20 25 30 35 40 45

Inte

nsi

ty

2θ

FTO

FTO

FTO

Cu2O

(111)

Cu2O

(200)Cu

Cu2O

0

200

400

600

800

1000

1200

1400

1600

20 25 30 35 40 45

Inte

nsi

ty

2θ

Cu2O

Cu2O

(111)

FTO

FTO

FTO

Au

Cu

Cu2O

(200)

Au

36


substrate (top) and on a gold coated FTO coated glass substrate (bottom). This

sample used electrolyte 2

F

TO

0

200

400

600

800

1000

1200

1400

1600

1800

20 25 30 35 40 45

Inte

nsi

ty

2θ

Cu2O

(111)

Cu2O

(200)

Cu2O Cu

FTO

FTO

FTO

0

200

400

600

800

1000

1200

1400

1600

1800

20 25 30 35 40 45

Inte

nsi

ty

2θ

Cu2O

(111)

Cu2O

(200)Cu2O

Au

Au

FTO

FTO

FTO

37

Transmission Electron Microscopy

TEM was used to determine whether there was epitaxial growth occurring at the

interface of the gold nano-islands and Cu2O, and a film grown using E2. First a sliver

of the film was cut from the sample using a focused ion beam (FIB). This was performed

by Evans Analytical Group (EAG) in an FEI Strata 400 Dual Beam FIB/SEM. During

this process the ion beam was used to cut away trenches in the sample so that a narrow

section could be removed and attached to a sample holder using platinum and thinned

using the same ion beam, the thinning is necessary for the TEM analysis. Once the

sample was prepared it was analyzed using the FEI Tecnai TF-20 FEG/TEM operated

at 200kV in bright-field (BF) TEM mode and high-resolution (HR) TEM mode. The

sample was aligned and images were taken. Two of these images are shown in figure

3.8 and 3.9. Both figures are bright field, high resolution TEM image.

38

Figure 3.8 HR-TEM showing the cross section of the electrodeposited Cu2O on

gold nano-islands and FTO coated glass. The platinum is present from the process

of cutting the sample using FIB.

Figure 3.8 was taken at a magnification of 39KX and shows the full cross section

of the film that was produced. The top layer is platinum that was deposited during the

FIB process, followed by the electrodeposited layer of Cu2O, below which are the gold

nano-islands. The gold nano-islands are on top of the commercially prepared FTO

coated glass. In this image the grains are faceted and are growing vertically in the desired

columnar shape.

39

Figure 3.9 HR-TEM magnified image of a gold particle that has Cu2O

electrodeposited on top. The black box highlights the region where a fast Fourier

transform was taken.

Figure 3.9 is a high magnification image at 360KX. This image focuses on the

interface between the gold particle and the Cu2O, and at this magnification the lattice

fringes are apparent proving that there is high quality crystal growth of the Cu2O.

DigitalMicrograph was then used to get more information from the TEM images, using

the fast fourier transform (FFT) function. The white square shown in figure 3.9 is the

area highlighted in figure 3.10 shown below. In this region both Cu2O and gold are

represented and they are shown in the figure using arrows. These spots are reminiscent

40

of the diffraction pattern that is shown in figure 3.2 a5 and b5 where there is a large spot

and a small spot very close to one another. The orientation of the spots also matches the

diffraction pattern of the six spots that surround the beam blocker in the image from

figure 3.2 a5, which indicates that the zone axis in that region of the film is [11̅1̅].

Figure 3.10 Fast Fourier transform of the area highlighted in figure 3.9. The

arrows highlight the spots that represent Cu2O and Au in the pattern.

The next area on figure 3.9 that was analyzed was the center of the diagonal edge

of the Au particle. An FFT was taken from this region and then an inverse FFT was

taken to get a higher magnification image of the same region, this can be seen in figure

3.11. This images shows the interface between the gold particle which is the

41

Figure 3.11 Inverse fast Fourier transform of the interface between a gold particle

(left side) and Cu2O film (right side) this is from the boxed area highlighted in

figure 3.9

the darker region on the left, and the Cu2O film, which is the lighter region on the right.

In the image the lattice fringes can be seen running smoothly between the interface, from

the gold particle into the Cu2O film. Also in this figure the FFT from either side of the

interface is overlaid on the corresponding region. In the FFT for the Cu2O region the

diffraction spots have been labeled with Miller indices. This pattern has a zone axis of

[11̅0]. The gold has the same orientation and would be indexed the same way. Using

figure 3.11 the fringe spacing was measured on the Cu2O to be 0.253 nm, and this is

42

consistent with the known Cu2O lattice spacing of the {111} plane. Finally, the interface

between the gold and Cu2O in figure 3.11 is in the (111) plane, this is known because it

is perpendicular to the (111) Miller index from the Cu2O FFT. This would be one of the

lowest energy surfaces of gold.

Solar Simulation

A Newport model solar simulator was used to obtain electrical properties from

the Cu2O films that were electrodeposited from E2 for one hour at 1.5 V. As mentioned

earlier there is some work that uses tin oxide (SnO2) as the n-type material for Cu2O

solar cells. This characterization takes the electrodeposited films and tests them in a

solar simulator using the FTO substrate as the n-type material as well as the top contact.

A device was also made using Cu2O deposited onto gold on FTO. For the simulation a

back contact of Au was sputtered on with a thickness of 60nm. In table I, in section 1.3,

Au is a material that has been used as the back contact to a Cu2O solar cell, The solar

simulation was programed to run from -1 volt to 1 volt. Two runs were completed for

each sample: one run with the solar simulation light on and one without the light. The

simulation results are shown in figures 3.12 and 3.13.

43

Figure 3.12 Solar simulation data for electrodeposited Cu2O on an FTO substrate

in light (orange) and in dark (blue).

Figure 3.13 Solar simulation data for electrodeposited Cu2O on an Au/FTO

substrate in light (blue) and in dark (green).

y = 0.0002x - 4E-07

y = 0.0017x + 5E-06

-2.00E-03

-1.50E-03

-1.00E-03

-5.00E-04

0.00E+00

5.00E-04

1.00E-03

1.50E-03

2.00E-03

-1.5 -1 -0.5 0 0.5 1 1.5

Curr

ent

(A)

Voltage (V)

Dark

Light

y = 0.0429x + 7E-06

y = 0.0448x - 0.0001

-0.015

-0.01

-0.005

0

0.005

0.01

0.015

-0.3 -0.2 -0.1 0 0.1 0.2 0.3

Cu

rren

t (A

)

Voltage (V)

Dark

Light

44


From the results of the XRD and SEM we have further concluded that E2 is the

best electrolyte for this project. When used in combination with the gold nano-islands

the SEM images (figure 3.2) show that the surface is faceted as expected from the Au-

Cu2O nanoparticles, and the dendritic growth that is common in electrodeposited Cu2O

does not occur under the influence of the Au seed particle. With the data from the XRD

the E2 film on the seed particle substrate was found to be without the trace amount of

copper that was in the films when it was deposited directly on FTO. E1 did not show the

elimination of copper with the addition of gold.

When TEM analysis was used on a film of electrodeposited E2 on gold nano-

islands the growth of the Cu2O on the gold nano-islands was found to be epitaxial. The

cross sectional image showed columnar shaped grains stemming from gold particles.

With further analysis, using the fast Fourier transform and inverse fast Fourier transform

features in the DigitalMicrograph software, it has been determined that the diffraction

pattern in the Cu2O and the Au are similar. There is some shifting of the pattern, but the

orientations of the two materials is essentially matched, further proving that the gold

nano-islands are seeding the growth of the Cu2O films during the electrodeposition

process.

Finally the solar simulation on the p-type electrodeposited Cu2O and n-type FTO

did not result in any photovoltaic properties. This is illustrated by the plot that runs

through the origin for each of the samples, in both light and dark. The two types of film,

Cu2O/Au/FTO and Cu2O/FTO showed markedly different photoconductivity properties.

The sample with an Au substrate had an increase in slope with the light on, compared

45

to the run with the light off; the increase was 10 fold, from 0.0002 Ω-1 with the light off

to 0.0017 Ω-1 with the light on. There was no difference in the slope for the sample

without Au between the Cu2O and FTO layers. This is believed to be due to the

conductivity of the film without Au, the slope for I-V plots for the Cu2O/FTO samples

show the same slope, which is about 0.044 Ω-1. This is 22 Ω compared to the resistance

of the Cu2O/Au/FTO under light, which is 588 Ω. The Cu2O is more resistant, which

means that the photoconductivity properties of this film are more apparent compared to

the more conductive film (Cu2O/FTO). The resistivity is calculated using equation 5

where ρ is the resistivity, R is the resistance, A is the area of the film, and t is the film

thickness.

𝜌 = 𝑅𝐴/𝑡 (5)

The resistivity of the film of Cu2O on Au/FTO in dark was calculated to be

3.36x108 Ωcm. This is larger than the resistivity of other Cu2O films that can range from

76 Ωcm to 2000 Ωcm.[17,47,55,61,68] From the Olsen paper, the Cu2O is converted from Cu

metal and these samples are more strongly reduced showing a much lower resistivity

value.[17] Based on the calculated resistivity, the hole concentration was calculated using

equation 6, where p is the hole concentration, sigma is the conductivity (inverse

resistivity), q is the charge of an electron, μh is the hole mobility.

𝑝 = 𝜎/𝜇ℎ𝑞 (5)

For this calculation a few assumptions were made, the first is that n equals p and the

mobility is equal to silicon. The carrier concentration was calculated to be 8.06x107.

46

CHAPTER 4

4. Optical Properties of Cuprous Oxide

4.1 Background of Bandgap Estimation

The standard bandgap that is reported for Cu2O is 2.0 eV, however there are

some conflicting reports in the literature. The bandgap of Cu2O has been report ranging

from 1.7 to 2.7 eV. These reported numbers and the authors that reported them

Table II Summary of reported bandgaps of Cu2O and the authors that reported the

bandgaps.[18,20,47,48,51,59,61,63,77-79]

Author Reported Bandgap

(eV) Uihlein 2.172

Papadimitriou 2.3

Golden 2.1

Balamurugan 2.33-2.61

Georgieva 2.38

McShane 1.9

Akhavan 1.70-2.7

Gu 2.18

Hussain 2.19

Gupta 2.44

Jeong 1.70-2.53

are shown in Table II. This thesis is based on the idea the Cu2O is a good candidate for

the top cell of a tandem device primarily because the bandgap is frequently reported

around 2 eV. Therefore, it was important to confirm the bandgap of the material that

47

was the basis for this work. UV-Vis spectrometry and the Tauc method were used to put

together a Tauc plot for Cu2O films grown on FTO and Au/FTO.

Cu2O Band Structure

From a paper by Heinemann et al, the band structure of Cu2O has been calculated

using the density functional theory (DFT). This result is shown in figure 4.1, the band

structure is shown in the top image and the bottom image shows the corresponding

Brillouin zones for Cu2O.[80] In the shaded region around the Γ-point of the band

structure the two parabolic lines represent the valence band (bottom) and conduction

band (top). The gap between these two bands is the bandgap of Cu2O and in this image

it is at about 2.2 eV. Just above the conduction band is another parabolic band and the

gap between this band and the valence band is about 2.6 eV. These two gaps are pictured

again in figure 4.2, but in this case the only point from the Brillouin zones shown is the

Γ-point. In the image it shows the first gap at 2.17eV and the second gap, wider by 0.45

eV, at 2.62 eV. These two bandgaps are described as the direct bandgap (2.17 eV) and

the optical gap (2.62 eV)[81] and explains why there is a wide range of bandgaps reported

in table II.

48

Figure 4.1 The band structure of Cu2O and density of states from DFT

calculations (above) and the associated Brillouin zones (below) [80]

49

Figure 4.2 Energy-band diagram of Cu2O around the Γ-point.[81]

4.2 Tauc Method

To determine the bandgap, the Tauc method was used. This method was

developed by Tauc et al when exploring the optical properties of amorphous germanium.

The method takes the optical absorbance and plots it against the energy to determine the

bandgap. This method can be used to find the bandgap for any semiconductor

material.[82]

The equation for determining the absorbance is shown in equation 7. In this

equation α is the absorption coefficient, h is Planck’s constant, ν is the photon’s

frequency, Eg is the bandgap, and A is the proportionality constant.

(α h ν) 1/n

= A ( h ν – Eg ) (7)

The value of n represents the type of transition, either allowed or forbidden and

either direct or indirect. The values for each of these scenarios is given in Table III.

50

Table III Possible transition types and the corresponding n values

Transition Type n

Direct allowed 1/2

Direct forbidden 3/2

Indirect allowed 2

Indirect forbidden 3

To perform the Tauc analysis the first step is to obtain absorption data from a

range of energies, both above and below the transition, then plot (α h ν) 1/n

versus h ν.

To determine the material’s bandgap transition each n value is used and the plot resulting

in a linear region stemming from or close to the x-axis most accurately describes the

bandgap transition.

4.3 Cu2O Data Collection

The first step to determining the bandgap of this work’s Cu2O film, was to get

the absorption data from a Thermo Fisher Scientific Evolution 300 UV-Vis

Spectrometer. The Cu2O samples that were made for this data collection were made

using the same process described in Chapter 2 and Chapter 3. Six samples were made

with two substrates; FTO coated glass, and Au coated FTO coated glass, and at three

different deposition times: 30 seconds, one minute, two minutes. The three sample

thicknesses were explored to see how the thickness of the film might affect the bandgap.

The one hour samples were not used for this part of the experiment because they were

too thick to get reliable absorption data. The baseline correction used for the samples

were either FTO coated glass for those samples deposited on FTO coated slides or Au/

FTO coated glass for the samples where Cu2O was deposited on Au/ FTO coated glass.

51

Once the absorption data was obtained Tauc method was then employed. To

calculate absorption coefficient (α) the following expression was used:

𝛼 = 2.303𝐴/𝐿 (8)

Where A is the absorption and L is the thickness. The thickness was estimated using the

Faraday’s Law shown below as equations 9:

𝑇 = 𝐼 ∗ 𝑡 ∗ 𝐴 ∗ 10000/(𝑛 ∗ 𝐹 ∗ 𝜌 ∗ 𝑆) (9)

Where T is the thickness in μm, I is the current in Amps, t is the time in seconds, A is

the atomic weight in grams/mol, n is the valence of the dissolved metal, F is Faraday’s

constant F = 96,485.309, ρ is the density in grams/cm3, S is the deposition surface area

in cm2 and 10,000 is the constant to convert cm to μ.[83] This absorption coefficient was

plugged into equation 7. This was then plotted versus h ν and the different n values were

tested to determine the type of bandgap.


Using the Tauc plots the bandgap of the material can be extrapolated by taking

the vertical section of the graph and continuing it down to intersect with the x-axis, and

the energy value at the intercept is the bandgap. The bandgaps that were found using

this method were fairly consistent for each of the samples including different thicknesses

and from the FTO substrate sample to the Au/FTO sample. For the FTO/Cu2O samples

52

shown in figure 4.3, the slope of this section, illistrated with a thick black line, intercepts

the x-axis at about 2.6 eV for 30 second and one minute deposition times and 2.5 eV for

two minute deposition time. For the FTO/Au/Cu2O samples, shown in figure 4.4, the

black line intercepts the x-axis at about 2.4 eV for one minutes, and 2.5 eV for two

minute and 30 seconds.

Figure 4.3 Tauc plot for the electrodeposited Cu2O on FTO coated glass, for

deposition times of 30 seconds, 1 minute, and 2 minutes.

The the Tauc plot slope for the one minute depostion of Cu2O on Au/FTO sample

was calculated to be about 1.1x1014. The slope value is related to the density of states,

and the steeper the slope the flatter the band edge and the larger the effective electron

mass. Gallium arsenide (GaAs) has a more shallow Tauc plot slope, calculated from a

paper by Aspnes et al, to be about 4x109.[84] Compared to Cu2O the band edge is very steep

0.0

0.5

1.0

1.5

2.0

2.5

3.0

1 1.5 2 2.5 3

(αh

ν)2

(eV

2cm

-2)

x1

013

Photon Energy (eV)

2 min

1 min

30 sec

53

and the electron and hole effective mass is very low. ZnO was found to have a Tauc plot slope

of 1x1012,[85] so the effective mass is larger than that of GaAs. The energy level of Cu2O is shown

in figure 4.1 to be relatively flat compared to GaAs and ZnO, and the comparison of the Tauc

slopes for these materials confirms this comparison. Therefore, the effective electron mass for

Cu2O is large.

The Tauc plots showed that the films that resulted from the electrodeposition

onto FTO and the films that resulted from the electrodeposition onto Au/FTO did have

some differences in their absorption properties. The samples shown in figure 4.3 have

an urbach tail that is the result of scattering of light from the surface of the material. In

a standard Tauc plot there should be a sharp absorption edge where the plot goes from

a flat line at zero absorption to a vertical line. The FTO/Au samples have a Tauc plot

closer to the standard Tauc plot as shown in figure 4.4. The scattering on the Cu2O/FTO

samples is due to surface roughness. This roughness is apparent on the films when

looking at it with the naked eye because the surface has a milky color to it. The SEM

images in Chapter 2 show large faceted grains, these facets produce the scattering. The

SEM images for the Cu2O/Au films show much smaller grains that have smaller facets,

so the scattering is less intense. One area that is unique for the Cu2O/Au samples is the

absorption hump at about 1.5 eV, this is believed to be due to the gold on the substrates.

54

Figure 4.4 Tauc plot for the electrodeposited Cu2O on gold coated FTO coated

glass, for deposition times of 30 seconds, 1 minute, and 2 minutes.

Another theory as to why there is a tail for the FTO/Cu2O samples is that there

are two bandgaps represented by these Tauc plots. On the Tauc plot for the two-minute

deposition for the FTO/Cu2O there are two distinct flat sections. Based on the band

structure information in section 4.1.1 there are two gaps to consider, the first is the

optical bandgap, which has a width of about 2.6 eV. This is the bandgap that is

represented in each of the Tauc plots. The second gap is also direct but the transition is

forbidden[80,81] and has a width of 2.17eV. This gap is only shown in the Tauc plots from

the FTO/Cu2O samples. The line drawn for the absorption edge, identified using a

thinner black line on figure 4.3, intercepts the x-axis at about 2.1 eV. The Au/FTO films

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

1 1.5 2 2.5 3

(αh

ν)2

(eV

2cm

-2)

x1

013

Photon Energy (eV)

2 min

1 min

30 sec

55

do not show the second bandgap at 2.1 eV and that is because this film is more n-type

than the Cu2O film on FTO. An n-type semiconductor has more electrons in the

conduction band that means, in this case, the conduction of electrons will only take place

in the wider of the two bands.

Based on the findings from the Tauc plot, the bandgap of electrodeposited Cu2O

on FTO coated glass is too wide for the purposes of a tandem device. However, there is

a decrease in the bandgap under the influence of gold, which also resulted in the better

Tauc plot showing limited amounts of scattering.

56

CHAPTER 5

5. n-Type Cu2O

5.1 Background of n-Type Cu2O

Cuprous oxide is intrinsically a p-type material meaning that holes are the

primary carrier and electrons are the secondary carrier. One of the goals of this research

was to look into the fabrication of n-type Cu2O that can be used in the formation of a

homojunction solar cell. The existence of an n-type Cu2O has been discussed and could

be the result of oxygen vacancies.[41,49] There has also been work that shows that n-type

Cu2O can be formed by doping the film with excess copper. This doping is accomplished

through thermal diffusion. The idea is that this dopant of Cu ions will remove Cu

vacancies and introduce Cu interstitials and Cu on O sites, changing the material from

a p-type to an n-type.[19,86]

Han, et al worked on homojunction Cu2O solar cells, where the deposition was

carried out with two different pH levels, below 8.0 and above 9.0 for n-type Cu2O and

p-type Cu2O respectively.[19,86] Other works reported the effect that pH has on

controlling the carrier type for Cu2O, where the pH of the electrolyte was brought down

to 4.9 for electrodeposition of n-type Cu2O.[18,44,86] It was reported by Scanlon and

Watson that intrinsic n-type defects in Cu2O do not result in n-type behavior.[40] This

research focused on the work from Fernando, et al. Their group boiled copper plates in

a copper sulfate to convert the Cu to n-type Cu2O.[42] The results from this paper show

an n-type Cu2O. The goal here was to follow this research to determine if n-type Cu2O

57

could be obtained using this method. The substrate was modified to see if the result

would yield an n-type Cu2O film that could be used in a homojunction solar cell and

part of a tandem solar cell. The modifications consisted of using deposited copper on

glass and then converting it to Cu2O instead of converting a copper plate. This was to

allow for a solar cell that light could be transmitted through if it is not absorbed.

5.2 Experimental Reagents

All chemicals that were used for this work were as follows: (i) copper (II) sulfate

pentahydrate, greater than 98.0% purity, purchased from Sigma Aldrich (ii) sulfuric acid

95-98% pure, purchased from Sigma Aldrich; (iii) acetone, ACS grade, purchased from

Fisher Scientific.

5.3 Substrate Preparation

Three different types of samples were explored for the conversion of Cu to Cu2O.

The first substrate is the most common for the boil method, a solid piece of Cu, in this

case a thin Cu sheet was used. The next substrate that was used was electrodeposited Cu

onto FTO coated glass. The final substrate was sputtered Cu onto either glass or FTO

coated glass.

Copper Sheet

The first step was to duplicate the experimental procedure from Fernando, et al

by converting copper metal to Cu2O. A thin copper sheet was used as the substrate. This

58

sheet was 71nm thick and was cut down to a 2 by 5 cm sheet. The copper sheet was then

cleaned in an ultrasonicator three times using 2 vol % Micro-90 in DI water, DI water,

and finally acetone.

Electrodeposition of Copper

Electrodeposition of copper was also explored because a glass substrate, which

is ideal for the tandem solar architecture, could be used. A rectangular piece of FTO

coated glass was cleaned using the same procedure described above. Once cleaned the

substrate was attached in a two electrode system with a platinum counter electrode. Both

of these electrodes were submerged in the electrolyte and connected to a voltage source.

The electrolyte was a 0.05 M copper (II) sulfate pentahydrate in DI water. The solution

was brought to a pH between one and three using sulfuric acid. This deposition was run

for one minute at five volts.

Sputter Coating of Copper

Another method that was used to deposit the copper was sputter coating. The

substrate for this method was a thin glass slide or FTO coated glass, and both were

cleaned prior to the deposition using the same method described in section 5.2.1. For

this method the coater that was used for the gold deposition from Chapter 3 was used

but the gold target was replaced by a copper target (99.99% 57mm dia x 0.1mm thick,

from EMS) a number of different thicknesses were deposited (20, 30, 50, 85, 125nm).

59

5.4 Conversion from Cu to Cu2O

The next step of this experimental procedure was to take each of the three types

of thin films of copper and convert it to Cu2O. The substrates were clipped to Teflon

covered alligator clips that were attached to copper wire. The wire was then draped over

the edge of a beaker and submerged into a boiling solution of 10-3 M copper sulfate

pentahydrate in DI water. The substrate was left in the solution for one hour. During this

hour if the solution level dropped the water was replaced.[42] After an hour the film was

rinsed with DI water. Each of the substrates were boiled in individual beakers.


The films of Cu2O were characterized to determine whether they were n-type or

p-type by performing a hot probe test and a solar simulation with the p-type

electrodeposited Cu2O. Next they were analyzed using SEM to compare the growth of

the three different films. Finally, XRD was used to determine if the copper had convert

to Cu2O completely and what if any other phases were present.

Hot Probe Test

One way to determine if the deposited film resulted in n-type is a hot probe test.

This test was performed using a multi-meter set to read the current. The pins of the

multi-meter were set on the surface of the film one centimeter apart, one of the pins was

heated and the current was observed. If the positive pin is heated and the current reads

positive then the film is n-type, and if it reads negative then the film is p-type.

60

Solar Simulation

A Newport model solar simulator was used to determine if the p-type and n-type

Cu2O would work in as solar cell. The n-type process that was used for this testing was

the sputtered copper because it was the most consistent film. To perform this test three

samples were made: the first was n-type Cu2O deposited onto FTO followed by

electrodeposited p-type Cu2O, next was electrodeposited Cu2O deposited onto FTO

followed by the n-type Cu2O, and last was electrodeposited Cu2O deposited onto Au

and FTO followed by n-type Cu2O. All three devices had a back contact of 60nm thick

Au. The simulation was completed using from -1 to 1 volts.

SEM

Scanning electron microscopy was used to understand the conversion of the

copper to Cu2O. First images were taken of the copper sheet and the sheet after boiling.

This comparison is shown in figure 5.1 and 5.2. Next the images of electrodeposited

copper boiled in the copper sulfate were taken using the SEM, this is shown in figure

5.3. The final image was taken of the sputtered copper converted to Cu2O in figure 5.4.

61

Figure 5.1 SEM image of a copper sheet

Figure 5.2 SEM image of a copper sheet that has been boiled in copper sulfate to

convert it to Cu2O

62

Figure 5.3 SEM image of electrodeposited copper on FTO coated glass that has

been converted to Cu2O by boiling in copper sulfate.

Figure 5.4 SEM image of sputtered copper on glass that has been converted to

Cu2O by boiling in copper sulfate.

63

XRD

X-ray diffraction was used to determine phases of copper oxide that are present,

and how much, if any, copper remained from the boiling process. All three types of

substrates were analyzed using an X’Pert PANalytic XRD. The first is shown in figure

5.5, showing the pattern resulting from the boiled copper sheet. The next pattern, figure

5.6, shows the results from the electrodeposited copper on FTO coated. The final pattern,

figure 5.7, is for the sputter coated copper on glass.

Figure 5.5 XRD data of Cu2O resulting from a copper sheet boiled in copper

sulfate.

0

500

1000

1500

2000

2500

3000

25 30 35 40 45 50 55

Inte

nsi

ty

2θ

FTO

FTO

FTO

Cu2OCu2O

FTO

64

Figure 5.6 XRD data of Cu2O resulting from electrodeposited copper onto FTO

coated glass and then boiled in copper sulfate.

Figure 5.7 XRD data of Cu2O resulting from a sputtered copper onto glass and

then boiled in copper sulfate.

0

10000

20000

30000

40000

50000

60000

70000

80000

25 30 35 40 45 50 55

Inte

nsi

ty

2θ

Cu2O Cu2OCu

Cu

0

100

200

300

400

500

600

700

800

25 30 35 40 45 50 55

Inte

nsi

ty

2θ

Cu2OCu2O

Cu2O

65


The boiled copper sheet method resulted in a film that was flaking and uneven.

In XRD the results were inconclusive. Since the substrate is copper it was impossible to

determine using this technique if the top layer was 100% Cu2O, though there was

conversion of the copper to Cu2O it is unclear from the XRD results if the top layer of

material contained any copper along with the Cu2O. The SEM results showed a growth

of distinct grains compared to the SEM image from figure 5.1, which showed the copper

sheet before boiling.

The electrodeposition resulted in even films that were consistent in their

replicability. The one issue that arose was if the copper film was exposed to air for any

length of time after the electrodeposition and before boiling, then the film would oxidize

unevenly and result in a flaking and uneven Cu2O film after boiling. The XRD results

showed that there was full conversion from Cu to Cu2O.

Initially a glass microscope slide was used for the sputter coated films. These

were inconsistent. The best film resulted from the 125nm thick sputtered copper, the

thinner samples flaked off during the boiling process. The second attempt used FTO

coated glass, which resulted in a much more even and stable film. The XRD results from

the sputter coated sample show similar results to the electrodeposited Cu.

The hot probe test was inconclusive, the multimeter would swing from positive

to negative. The solar simulation results did not show any photovoltaic or photocurrent

response, though this could have been due to the quality of the films that were grown.

The sputtered copper film converted to Cu2O but did not fully cover the surface, so there

could have been pin holes or the p-type and n-type materials might not have made

66

enough of a connection to form the p-n junction. From these results it can be assumed

that the n-type characteristics that were presented from the paper by Fernando et al were

not reproduced for this work..

67

CHAPTER 6

6. Conclusions

This research is fueled by the idea that there are alternative energies such as solar

that have only just begun to be tapped into and are needed in order to alleviate the use

of destructive and limited energy source such as fossil fuels. The work to make

photovoltaic technologies cheaper and more efficient is the key to obtaining a larger

market. The biggest problem with efficiency is that energy loss is inevitable because of

the solar spectrum. There will always be losses, particularly with single junction solar

cells, because there is energy lost when a photon exceeds the bandgap and the photons

that have a lower energy than the band gap are not absorbed. Silicon has the best

efficiency for a single junction solar cell because it has a bandgap of 1.1 eV this bandgap

width balances the described losses, and therefore, silicon solar cells are dominating the

market at 21% efficient. Energy losses can be further minimized by stacking solar cells

so that more photons are absorbed. The use of silicon solar cells as the bottom layer of

this stack is a promising path to take. The overarching goal of this research was to

establish a wide band gap solar cell that could be used as the top cell of a tandem solar

cell. This top cell would have to be cheap and grown on the glass that encases the top

layer of the single junction silicon solar cell. Cu2O was the focus for this work as the

wide band gap absorber layer for the wide bandgap solar cell that can be used in a

tandem solar cell.

68

Electrodeposition was identified as the preferred process for the deposition of

Cu2O because it is a low cost process and results in even films that can be scaled to

manufacturing sizes. Two different electrolytes were explored. The two types were each

deposited using a two electrode system onto FTO coated glass and the grain growth was

explored by depositing the film for times from 30 seconds to one hour. The SEM images

showed that E1 resulted in a growth that was not well defined at one hour, E2 resulted

in grains that were a flowering dendritic shape. The XRD results showed that both films

were Cu2O but that there was a small amount of Cu present in each film.

The ideal grain shape for a thin film solar cell would be a columnar shape so that

the electrons can flow through the grain without interruption of grain boundaries, where

recombination of the electron and hole is common. The next step was to take the two

different films deposited for one hour and manipulate the growth so that the grain shape

was closer to the ideal columnar shape. This was achieved using the well-known

relationship between Cu2O and Au, and using the Au as a seed to grow the Cu2O. The

Au seed layer was deposited using a sputter coater, depositing a thin layer of Au, and

then baking the film to turn the thin film of Au into Au nano-islands on FTO coated

glass. This layering of Au nano-islands on FTO coated glass was then used as the

working electrode for the electrodeposition. The SEM results showed smaller grains and

that grain growth was much more controlled and the grains were densely packed. The

grains also showed a strong faceting. XRD showed that with the presence of Au nano-

islands, the copper that was growing with E2 was no longer present, but the copper was

still present for the E1 sample even with Au nano-islands. TEM images from a sample

that was cut out of the E2 sample on Au/FTO, showed that the Cu2O that was growing

69

on the Au was growing epitaxially, confirming that there is an orientation relationship

between the two layers. Finally, the E2 films grown on Au/FTO and FTO were subject

to electrical characterization and although photovoltaic properties were not present there

was an indication of photocurrent properties in the film grown on Au/FTO. This was

because the Cu2O grown on Au/FTO was more resistant compared to the film grown on

FTO, and therefore the photoconductivity was more obvious.

The optical properties for E2 on Au/FTO and FTO were explored due to the

importance of the bandgap estimation for this work and the lack in consistency in the

reported bandgap for Cu2O in the literature. The optical measurement was completed

using UV/Vis spectrometer to get the absorption data for three samples from each

substrate. Each sample had a different deposition time (30 seconds, one minute, and two

minutes). Using the Tauc method a Tauc plot was developed using the absorption data

and an estimation of the film thickness. This Tauc plot showed that the bandgap for

Cu2O is direct and allowed. Using the absorption edge the bandgap was found to be

around 2.6 eV for the FTO substrate sample and 2.5 eV for the Au/FTO substrate

sample. The two Tauc plots showed some significant differences. In the Au substrate

sample, the Tauc plot had an absorption edge that was very close to the x-axis unlike

the FTO substrate sample, which had a large tail before the absorption edge. This tail

could be a result of the scattering of light on the surface of the film, but there also appears

to be a second straight section just before the large absorption edge at 2.6 eV, which

intersects the x-axis at about 2.1 eV. Based on the literature there are two bandgaps for

Cu2O one at 2.64 eV and one at 2.17 eV, these two gaps can only be seen in the Tauc

plot for the sample deposited on FTO. The other area of difference comes from the Tauc

70

plot for the Au/FTO sample, there is a hump at 1.5 eV, which is believed to be from the

Au that is present in the film. The results of this measurement showed that the Au/FTO

films had less scattering and band gap around 2.5 eV while the FTO samples potentially

had two bandgaps one at 2.6 eV and the second at 2.1 eV.

The development of n-type Cu2O has been an area of interest for a homojunction

solar cell with p-type Cu2O. This is due to the idea that a homojunction solar cell has

the potential for a higher efficiency compared to a heterojunction solar cell. The method

for obtaining an n-type Cu2O for this research was from the work of Fernando et al.[42]

The goal of this chapter was to establish if this work was reproducible and could be

manipulated for use in the application being explored. The paper uses a copper plate and

boils it in a copper sulfate solution, this process was duplicated using a copper sheet.

This substrate is not ideal because the back and front contact have to be transparent for

use in a tandem cell, it was also difficult to determine if the film that was made was

100% Cu2O because there was always going to be copper present from the substrate.

The modification that was explored was depositing copper onto FTO and boiling this

thin film of Cu to transform it into Cu2O. This was completed first by electrodepositing

Cu onto FTO. The resulting Cu2O film was evenly coated but it scratched off easily.

Another problem with this film was that the electrodeposited Cu was not stable before

boiling, if left in air for only a couple of minutes the film would change color, become

flakey and would flake off into the boiling solution. The next deposition process of Cu

that was explored was sputter coating. Using a copper target, the FTO coated glass was

coated with 125nm thick layer and then boiled. This process resulted in the most reliable

film, it also scratched off easily but it did not flake into the solution. The XRD results

71

showed that each of these processes produced Cu2O and Cu was not present except in

the case where it was expected. The SEM results showed similar growth of Cu2O for the

electrodeposited and the sputter coated samples with slightly larger grains from the

sputter coated sample. The boiled copper sheet resulted in large grains of Cu2O. The

sputter coated sample was the most reliable of the three samples and would allow the

deposition of Cu2O onto an insulating surface as well as a conduction surface, which

will allow for further testing. The hot probe test was inconclusive so the carrier type was

not verified using this method. A solar cell was attempted using the electrodeposited p-

type from Chapters 2 and 3. Three solar cells were tested, FTO/n-type/p-type/Au,

FTO/p-type/n-type/Au, and FTO/Au/p-type/n-type/Au. This testing showed no

photovoltaic or photocurrent responses for any of the attempted devices.

72

CHAPTER 7

7. Future Work

The research completed for this thesis is just the tip of the iceberg for Cu2O thin

film fabrication and photovoltaic work. While the results that have come from this thesis

are interesting and insightful, it seems to indicate that Cu2O would not make for an ideal

material for the absorber layer of the top cell of a tandem device. There are some areas

of work that could be explored to improve upon these films for use in other applications.

7.1 p-Type Cu2O

In Chapters 2 and 3 the deposition process for Cu2O is described, along with

how the film microstructure was modified. This resulted in a more controlled film that

was fully densified. One area that would be interesting to delve into would be to change

the carrier concentration of the film. This would allow for manipulation of the depletion

region of the p-n junction. The carrier concentration was not determined or determinable

for these films because they were deposited onto a conductive substrate. One method

that would allow for carrier concentration testing would be the Cu2O film that resulted

from the sputtering of copper onto glass. The problem with the film developed by that

method for this research was that the film did not fully cover the glass after conversion

so it would require multiple layers of deposition.

From the attempted solar cells using FTO as the substrate and the n-type

material, it was determined that the Cu2O that was deposited onto Au/FTO had a

photoconductivity response. The current was very low, but it was shown from the work

73

by Du Pasquier et al, that the small photocurrent can be found for the photon energies

where absorption occurs for a material.[87] The set up for this characterization technique

is shown in figure 7.1.

Figure 7.1 Set up for photocurrent measurements[87]

The wavelength is picked through the monochromator where a fiber optic

refection probe is attached. The probe has a metal casing and it is electrically connected

to the sample’s surface with a gel electrolyte. A lock-in amplifier is required because

the signal is so low. The amplifier is attached to the metal casing of the probe and the

substrate contact that the sample is deposited on. This experiment will allow for

photocurrent measurements from the photon energies that are associated with the

absorption that was determined in Chapter 4.

74

7.2 n-Type Cu2O

In Chapter 5 it was determined that the Cu2O film that resulted from the boiling

method was not an n-type semiconductor. The carrier type was first tested with the hot

probe method but this was inconclusive, so the next step was to take the presumptive n-

type material and pair it with the p-type that was being grown using electrodeposition

described in Chapters 2 and 3. This test resulted in a film that did not show photovoltaic

properties or photocurrent properties. This leads to the belief that what was made by

boiling copper in copper sulfate did not lead to n-type Cu2O. The first step for

confirming this finding would be to increase the coverage of the Cu2O from boiled

sputtered Cu, this could be completed by repeating the sputtering and boiling once or

twice more. Another possible way of modifying this n-type material would be to use a

dopant in the boiling solution. This could result in a more reliable n-type Cu2O. There

is work indicating that Cu, fluorine (F), chlorine (Cl), and bromine (Br) might make for

stable dopants in Cu2O and that the doping of these elements would result in n-type

Cu2O.[19,45]

7.3 Solar Cell Fabrication

The solar cells that were created during the research of this process did not result

in any photovoltaic properties. There are quite a few paths that can be taken to improve

these results described in Chapter 3 and 5. The first would be to understand the p-type

material, Chapter 3 described research where the p-type Cu2O was tested on FTO to

determine if this p-n junction would result in a solar cell and the results indicated that it

would not. One concern for this is that the unmodified Cu2O on FTO grew as a film with

75

gaps between the grains, this could be mitigated by growing a thicker film. The growth

is limited by the resistivity of the film as it grows thicker, so when the surface of the

Cu2O is no longer the optimum area of growth, the growth should continue in the gaps

between the grains.

The modified Cu2O grown on Au/FTO might not have made a good enough p-n

junction because the interference with the gold between the two layers. Therefore

another area that could be further explored is the use of a well-established n-type

material that has been successfully paired with Cu2O, such as ZnO. Over the years ZnO

has been researched frequently as the n-type in Cu2O heterojunction solar cell, with

mixed results.[23,29,30,32-35,66,79] This would require a well-known deposition process for

ZnO so that the only variable factor would be the Cu2O, the effect of the thickness of

the film and the effect of the gold on electrical properties of the solar cell.

76

BIBLIOGRAPHY

[1] R. F. Service. Is It Time To Shoot For The Sun? Science. 309, 2005, 548-551.

[2] J. Potocnik. Renewable Energy Sources and the Realities of Setting an Energy

Agenda, Science. 315, 2007, 810-811.

[3] T. Conti, P. Holtberg, J. A. Beamon, S. Napolitano, A. M. Schaal, J. T. Turnure,

and L. Westfall. International Energy Outlook 2013 U. S. Energy Information

Administration, DOE/EIA-0484, 2013.

[4] "International Energy Statistics." 2013.Web.

<http://www.eia.gov/cfapps/ipdbproject/IEDIndex3.cfm?tid=2&pid=2&aid=12>.

[5] J. L. Sawin, C. Lins, E. Musolino, K. Petrichenko, W. Rickerson, K. Seyboth, J.

Skeen, B. Sovacool, F. Sverrisson, and L. E. Williamson. Renewables 2015 Global

Status Report REN21, 2015.

[6] Q. Schiermeier, J. Tollefson, T. Scully, A. Witze, and O. Morton. Energy

Alternatives: Electricity without Carbon, Nature. 454, 2008, 816.

[7] M. Rekinger, F. Thies, G. Masson, and S. Orlandi. Global Market Outlook for

Solar Power 2015-2019 Solar Power Europe, 2015.

[8] "Research Cell Efficiency Records." 02-25-2016 2016.Web.

<http://www.nrel.gov/ncpv/>.

[9] S. P. Philipps, A. W. Bett, K. Horowitz, and S. Kurtz. Current Status of

Concentrator Photovoltaic Technology Fraunhofer ISE/NREL, CPV Report 1.2, 2016.

[10] V. Vijayakumar, and D. P. Birnie. Optical and Electronic Simulation of Gallium

Arsenide/Silicon Tandem Four Terminal Solar Cells, Solar Energy. 97, 2013, 85-92.

[11] C. H. Seager. Grain Boundary Recombination: Theory and Experiment, Journal

of Applied Physics. 6 52, 1981, 3960.

[12] "CdTe Photovoltaic Cell Graphic." 2013.Web.

<http://www.nrel.gov/pv/images/graphic_cdte_cell.jpg>.

[13] C. H. Henry. Limiting Efficiencies of Ideal Single and Multiple Energy Gap

Terrestrial Solar Cells, Journal of Applied Physics. 51, 1980, 4494-4500.

[14] "Photovoltaic Cell Structure Basics." 08-19-2013 2013.Web.

<http://energy.gov/eere/energybasics/articles/photovoltaic-cell-structure-basics>.

http://www.eia.gov/cfapps/ipdbproject/IEDIndex3.cfm?tid=2&pid=2&aid=12

http://www.nrel.gov/ncpv/

http://www.nrel.gov/pv/images/graphic_cdte_cell.jpg

http://energy.gov/eere/energybasics/articles/photovoltaic-cell-structure-basics

77

[15] L. Fraas, and L. Partain. Solar Cells and their Applications. 2nd ed. Hoboken, NJ:

John Wiley & Sons Inc., 2010. Print. Wiley Series in Microwave and Optical

Engineering .

[16] D. Trivich. Photovoltaic Cells and Their Possible Use as Power Converters for

Solar Energy, Ohio Journal of Science. 5 53, 1953, 300-314.

[17] L. C. Olsen, F. W. Addis, and W. Miller. Experimental and Theoretical Studies of

Cu2O Solar Cells, Solar Cells. 7, 1982, 247-279.

[18] C. M. McShane, and K. Choi. Photocurrent Enhancement of n-Type Cu2O

Electrodes Achieved by Controlling Dendritic Branching Growth, Journal of the

American Chemical Society. 131, 2008, 2561-2569.

[19] K. Han, and M. Toa. Cu Diffusion Doping in Electrodeposition n-Type Cu2O,

216th ECS Meeting. Abstract #760, 2009, .

[20] N. Gupta, S. Rajandra, F. Wu, J. Narayan, C. McMillen, K. F. Poole, G. F.

Alapatt, D. Sulejmanovic, S. Hwu, M. Young, G. Teeter, and H. S. Ullal. Deposition

and Characterization of Nanostructured Cu2O Thin-Film for Potential Photovoltaic

Applications, Journal of Materials Research. 13 28, 2013, 1740-1746.

[21] M. Li, W. Wu, K. Liu, G. Hu, and H. Xu. Three-Dimensional Assembly and

Electrical Properties of Cu2O/ZnO Heterojunction via an Electrochemical Superfilling

Method, Electrochimica Acta. 71, 2012, 100-105.

[22] L. Chen. Review of Preparation and Optoelectronic Characteristics of Cu2O-

based Solar Cells with Nanostructure, Materials Science in Semiconductor Processing.

5 16, 2013, 1172-1185.

[23] T. Minami, T. Miyata, and Y. Nishi. Cu2O-based Heterojunction Solar Cells with

an Al-doped ZnO/Oxide Semiconductor/Thermally Oxidized Cu2O Sheet Structure,

Solar Energy. 105, 2014, 206-217.

[24] T. Minami, Y. Nishi, and T. Miyata. Efficiency Enhancement Using a Zn1-xGex-O

Thin Film as an n-Type Window Layer in Cu2O-Based Heterojunciton Solar Cells,

Applied Physics Express. 9, 2016, 052301.

[25] T. Minami, Y. Nishi, and T. Miyata. Heterojunction Solar Cell with 6%

Efficiency Based on an n-Type Aluminum-Gallium-Oxide Thin Film an p-Type

Sodium-Doped Cu2O, Applied Physics Express. 8, 2015, 022301.

[26] R. J. Iwanowski, and D. Trivich. The Influence of Hydrogen Ion Bombardment

on the Photovoltaic properties of Cu/Cu2O Schottky Barrier Solar Cells, Radiation

Effects Letters. 76, 1983, 87-92.

78

[27] J. Katayama, K. Ito, M. Matsuoka, and J. Tamaki. Performance of Cu2O/ZnO

Solar Cell Prepared by Two-Step Electrodeposition, Journal of Applied

Electrochemistry. 34, 2004, 687-692.

[28] M. Deo, S. Mujawar, O. Game, A. Yengantiwar, A. Banpurkar, S. Kulkarni, J.

Jog, and S. Ogale. Strong Photo-Response in a Flip-Chip Nanowire p-Cu2O/n-ZnO

Junction, Nanoscale. 3, 2011, 4706-4712.

[29] M. Deo, D. Shinde, A. Yengantiwar, J. Jog, B. Hannoyer, X. Sauvage, M. More,

and S. Ogale. Cu2O/ZnO Hetero-Nanobrush: Hierarchical Assembly, Field Emission

and Photocatalytic Properties, Journal of Materials Chemistry. 22, 2012, 17055-

17062.

[30] M. Zemzemi, and S. Alaya. Band Offset of the ZnO/Cu2O Heterojunction from ab

Initio Calculations, Superlattices and Microstructures. 64, 2013, 311-318.

[31] T. Minami, H. Tanaka, T. Shimakawa, T. Miyata, and H. Sato. High-Efficiency

Oxide Heterojunction Solar Cell Using Cu2O Sheets, Japanese Journal of Applied

Physics. 43, 2004, L917-L919.

[32] T. Minami, Y. Nishi, and T. Miyata. Effect of the Thin Ga2O3 Layer in n+-ZnO/n-

Ga2O3/p-Cu2O Heterojunction Solar Cells, Thin Solid Films. 549, 2013, 65-69.

[33] K. Akimoto, S. Ishizuka, M. Yanagita, Y. Nawa, G. K. Paul, and T. Sakurai. Thin

film deposition of Cu2O and application for solar cells, Solar Energy. 6 80, 2006, 715-

722.

[34] S. Noda, H. Shima, and H. Akinaga. Cu2O/ZnO Heterojunction Solar Cells

Fabricated by Magnetron-Sputtering Deposition Method Films Using Sintered

Ceramic Targets, Journal of Physics: Conference Series. 012027 433, 2012, 012027.

[35] M. Yang, L. Zhu, Y. Li, L. Cao, and Y. Guo. Asymmetric Interface Band

Alignments of Cu2O/ZnO and ZnO/Cu2O Heterojunctions, Journal of Alloys and

Compounds. 578, 2013, 143-147.

[36] R. Jayakrishnan. Photovoltaic Response of Cu2O/In2S3 Hetero-Structure Grown

on Cu Substrate, Materials Science in Semiconductor Processing. 6 16, 2013, 1608-

1612.

[37] C. Jayathialaka, V. Kapakils, W. Siripala, and S. Jayanetti. Improved Efficiency

of Electrodeposited p-CuO/n-Cu2O Heterojunction Solar Cell, Applied Physics

Express. 8, 2015, 065503.

[38] C. Chao, Y. Ohkura, T. Usui, and J. M. Weisse. Methods for Improving

Efficiencies of Cuprous Oxide Solar Cells, .

79

[39] H. S. Kim, J. W. Lim, S. J. Yun, M. A. Park, S. Y. Park, S. E. Lee, and H. C. Lee.

Fabrication and Characterization of Rapidly Oxidized p-Type Cu2O Films from Cu

Films and their Application to Heterojunction Thin Film Solar Cells, Japanese Journal

of Applied Physics. 52, 2013, 10MB17.

[40] D. O. Scanlon, and G. W. Watson. Undoped n-Type Cu2O: Fact or Fiction? The

Journal of Physical Chemistry Letters. 1, 2010, 2582-2585.

[41] W. Siripala, and J. R. P. Jayakody. Observation of n-Type Photoconductivity in

Electrodeposited Copper Oxide Film Electrodes in a Photoelectrochemical Cell, Solar

Energy Materials. 14, 1986, 23-27.

[42] C. A. N. Fernando, P. H. C. de Silva, S. K. Wethasinha, I. M. Dharmadasa, T.

Delsol, and M. C. Simmonds. Investigation of n-Type Cu2O Layers Prepared by Low

Cost Chemical Method for Use in Photovoltaic Thin Film Solar Cells, Renewable

Energy. 26, 2002, 521-529.

[43] C. M. McShane, W. P. Siripala, and K. Choi. Effect of Junction Morphology on

the Performance of Polycrystalline Cu2O Homojunction Solar Cells, The Journal of

Physical Chemistry Letters. 1, 2010, 2666-2670.

[44] Y. Hsu, J. Wu, M. Chen, Y. Chen, and Y. Lin. Fabrication of Homojunction Cu2O

Solar Cells by Electrochemical Deposition, Applied Surface Science. 354, 2015, 8.

[45] Q. Bai, W. Wang, Q. Zhang, and M. Tao. N-Type Doping in Cu2O with F, Cl, Br:

A First-Principles Study, Journal of Applied Physics. 11, 2012, 023709.

[46] L. Yu, L. Xiong, and Y. Yu. Cu2O Homojunction Solar Cells: F-Doped n-Type

Thin Film and Highly Improved Efficiency, Journal of Physical Chemistry. 119, 2015,

22803-22811.

[47] L. Papadimitriou, and N. A. Economou. Heterojunction Solar Cells on Cuprous

Oxide, Solar Cells. 3, 1981, 73-80.

[48] V. Georgieva, and M. Ristov. Electrodeposited Cuprous Oxide on Indium Tin

Oxide for Solar Applications, Solar Energy Materials and Solar Cells. 1 73, 2002, 67-

73.

[49] K. Han, and T. Meng. Electrochemically Deposited p-n Homojunction Cuprous

Oxide Solar Cells, Solar Energy Materials & Solar Cells. 93, 2009, 153-157.

[50] T. Minami, Y. Nishi, T. Miyata, and J. Nomoto. High-Efficiency Oxide Solar

Cells with ZnO/Cu2O Heterojunction Fabrication on Thermally Oxidized Cu2O

Sheets. Applied Physics Express. 4, 2011, 062301.

80

[51] S. Hussain, C. Cao, Z. Usman, Z. Chen, G. Nabi, W. S. Khan, Z. Ali, F. K. Butt,

and T. Mahmood. Fabrication and Photovoltaic Characteristics of Cu2O/TiO2 Thin

Film Heterojunction Solar Cell, Thin Solid Films. 522, 2012, 430-434.

[52] C. Tsai, C. Chen, P. Fei, and Y. Hsu. Novel Semiconductor-Liquid

Heterojunction Solar Cells Based on Cuprous Oxide and Iodine Electrolyte,

Electrochimica Acta. 167, 2015, 112-118.

[53] N. G. Elfadill, M. R. Hashim, K. M. Chahrour, M. A. Qaeed, and M. Bououdina.

The Influence of Cu2O Crystal Structure on the Cu2O/ZnO Heterojunction

Photovoltaic Performance, Superlattices and Microstructures. 85, 2015, 908-917.

[54] A. O. Musa, T. Akomolafe, and M. J. Carter. Production of Cuprous Oxide, a

Solar Cell Material, by Thermal Oxidation and a Study of its Physical and Electrical

Properties, Solar Energy Materials and Solar Cells. 3–4 51, 1998, 305-316.

[55] G. A. Adegboyega. An Investigation of Copper Oxide Films for Photovoltaic

Applications, Solar & Wind Technology. 3-4 2, 1985, 191-194.

[56] Z. Rosenstock, and I. Riess. Preparation of Oxide Thin Films by Controlled

Diffusion of Oxygen Atoms, Solid State Ionics. 136-137, 2000, 921-926.

[57] S. C. Ray. Preparation of Copper Oxide Thin Film by the Sol–gel-like Dip

Technique and Study of their Structural and Optical Properties, Solar Energy

Materials and Solar Cells. 3–4 68, 2001, 307-312.

[58] D. S. C. Halin, I. A. Talib, M. A. A. Hamid, and A. R. Daud. Characterization of

Cuprous Oxide Thin Films on n-Si Substrate Prepared by Sol-Gel Spin Coating,

Journal of Solid State Technology and Science. 1 16, 2008, 232-237.

[59] O. Akhavan, H. Tohidi, and A. Z. Moshfegh. Synthesis and Electrochromic Study

of Sol–Gel Cuprous Oxide Nanoparticles Accumulated on Silica Thin Film, Thin Solid

Films. 24 517, 2009, 6700-6706.

[60] D. S. C. Halin, I. A. Talib, and A. R. Daud. The Effect of Spin Coating Rate on

Morphology and Optical Properties of Cuprous Oxide Thin Film Prepared by Sol-Gel

Technique, Journal of the Australian Ceramic Society. 1 46, 2010, 41-45.

[61] T. D. Golden, M. G. Shumsky, Y. Zhou, R. A. VanderWerf, R. A. Van Leeuwen,

and J. A. Switzer. Electrochemical Deposition of Copper (I) Oxide Films, Chemistry

of Materials. 8, 1996, 2499-2504.

[62] L. Wan, Z. Wang, Z. Yang, W. Luo, Z. Li, and Z. Zou. Modulation of Dendrite

Growth of Cuprous Oxide by Electrodeposition, Journal of Crystal Growth. 312,

2010, 3085-3090.

81

[63] Y. Gu, X. Su, Y. Du, and C. Wang. Preparation of Flower-Like Cu2O

Nanoparticles by Pulse Electrodeposition and Their Electrocatalytic Application,

Applied Surface Science. 256, 2010, 5862-5866.

[64] L. C. Liau, Y. Lin, and Y. Peng. Fabrication Pathways of p-n Cu2O Homojunction

Films by Electrochemical Deposition Processing, The Journal of Physical Chemistry.

50 117, 2013, 26426-26431.

[65] J. Xue, Q. Shen, W. Liang, X. Liu, L. Bian, and B. Xu. Preparation and

Formation Mechanism of Smooth and Uniform Cu2O Thin Films by Electrodeposition

Method, Surface and Coatings Technology. 216, 2013, 166-171.

[66] Q. Li, M. Xu, H. Fan, H. Wang, B. Peng, C. Long, and Y. Zhai. Dielectric

Properties Investigation of Cu2O/ZnO Heterojunction Thin Films by

Electrodeposition, Materials Science and Engineering: B. 8 178, 2013, 496-501.

[67] S. Laidoudi, A. Y. Bioud, A. Azizi, G. Schmerber, J. Bartringer, S. Barre, and A.

Dinia. Growth and Characterization of Electrodeposited Cu2O Thin Films,

Semiconductor Science and Technology. 28, 2013, 115005.

[68] M. Majumder, I. Biswas, S. Pujaru, and A. K. Chakraborty. Cuprous Oxide Thin

Films Grown by Hydrothermal Electrochemical Depostion Technique, Thin Solid

Films. 589, 2015, 741-749.

[69] R. M. Brady, and R. C. Ball. Fractal Growth of Copper Electrodeposits, Nature.

5965 309, 1984, 225-229.

[70] C. M. Lopez, and K. Choi. Electrochemical Synthesis of Dendritic Zinc Films

Composed of Systematically Varying Motif Crystals, Langmuir. 22, 2006, 10625-

10629.

[71] A. Osherov, C. Zhu, and M. J. Panzer. Role of Solution Chemistry in Determining

the Morphology and Photoconductivity of Electrodeposited Cuprous Oxide Films,

Chemistry of Materials. 25, 2013, 692-698.

[72] W. Zhoa, W. Fu, H. Yang, C. Tian, M. Li, L. Yixing, L. Zhang, Y. Sui, X. Zhou,

H. Chen, and C. Zou. Electrodeposition of Cu2O films and Their Photoelectrochemical

Properties, Crystal Engineering Communications. 13, 2011, 2871-2877.

[73] E. W. Bohannan, M. G. Shumsky, and J. A. Switzer. Epitaxial Electrodeposition

of Copper (I) Oxide on Single-Crystal Gold (100), Chemistry of Materials. 11, 1999,

2289-2291.

[74] M. E. Messing, K. Hillerich, J. Johansson, K. Deppert, and K. A. Dick. The Use

Of Gold For Fabrication Of Nanowire Structures, Gold Bulletin. 3 42, 2009, 172-188.

82

[75] K. Nakaoka, and K. Ogura. Electrochemical Preparation of p-Type Cupric and

Cuprous Oxides on Platinum and Gold Substrates from Copper (II) Solutions with

Varous Amino Acids, Journal of The Electrochemical Society. 11 149, 2002, C579-

C585.

[76] W. Wang, L. Lyu, and M. H. Huang. Investigation of the Effects of Polyhedral

Gold Nanocrystal Morphology and Facets on the Formation of Au-Cu2O Core-Shell

Heterostructure, Chemistry of Materials. 23, 2011, 2677-2684.

[77] C. Uihlein, D. Frohlich, and R. Kenklies. Investigation of Exciton Fine Structure

in Cu2O, Physical Review B. 6 23, 1981, 2731-2740.

[78] B. Balamurugan, and B. R. Mehta. Optical and Structural Properties of

Nanocrystalline Copper Oxide Thin Films Prepared by Activated Reactive

Evaporation, Thin Solid Films. 396, 2001, 90-96.

[79] Y. S. Jeong, H. Kim, and H. S. Lee. Growth and Characterization of p-Cu2O/n-

ZnO Nanorod Heterojunctions Prepared by a Two-step Potentiostatic Method, Journal

of Alloys and Compounds. 573, 2013, 163-169.

[80] M. Heinemann, B. Eifert, and C. Heiliger. Band Structure and Phase Stability of

the Copper Oxides Cu2O, CuO, and Cu4O3, Physical Review B. 87, 2013, 115111.

[81] B. K. Meyer, A. Polity, D. Reppin, M. Becker, P. Hering, P. J. Klar, T. Sandar, C.

Reindl, J. Benz, M. Eickhoff, C. Heiliger, M. Heinemann, J. Blasing, A. Krost, S.

Shokovets, C. Muller, and C. Ronning. Binary Copper Oxide Semiconductors: From

Materials Toward Devices, Physica Status Solidi B. 8 249, 2012, 1487-1509.

[82] J. Tauc, R. Grigorovici, and A. Vancu. Optical Properties and Electronic

Structure of Amorphous Germanium, Phys Status Solidi. 15, 1966, 627-637.

[83] "Faraday's Law." Electronics Inc. A California Corporation. 2013.Web.

<http://www.electrolytics.org/faradaysLaw.html>.

[84] D. E. Aspnes, S. M. Kelso, R. A. Logan, and R. Bhat. Optical Properties of

AlxGa1-xAs, Journal of Applied Physics. 60, 1986, 754.

[85] D. P. Birnie, and J. B. Coulter. Unpublished .

[86] W. Wang, D. Wu, Q. Zhang, L. Wang, and M. Tao. pH-Dependence of

Conduction Type in Cuprous Oxide Synthesized from Solution, Journal of Applied

Physics. 107, 2010, .

[87] A. Du Pasquier, Z. Duan, N. Pereira, and Y. Lu. Cuprous Oxide Solution

Preparation and Application to Cu2O-ZnO Solar Cells, ECS Transactions. 28, 2010,

179-190.

http://www.electrolytics.org/faradaysLaw.html

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